Opening principles itself will not save you from need to know particular opening lines, typical plans and tactical patterns of opening variations you play.
And playing opening you want to learn in blitz is very useful way how to "feel" the opening. It`s generally aknowledged by strong players, including GMs.
I think there are many of them because they're catering to their audience... i.e. amateurs love to read and study openings.
General opening ideas/lines are also much easier to explain and watch in general... actual study requires a lot of time and effort from the student... a video format doesn't facilitate this very well.
Opening knowledge is very important to survive at any level of chess, though of course, the amount of actual knowledge you need varies a lot according to your level of play.
Besides, it's easier to convey (I don't say to understand/use) than other kinds of technical knowledge, and it also appeals more to many players, because they can find an immediate use for it...
It would probably be more useful to analyse class players games and explain the roots of their mistakes, but it's much more difficult to do, than delivering a packaged opening variation. 
To be fair, I've noticed a lot of titled players offer interesting training exercises and game analysis rather than raw opening stuff in their blogs.
hmmm, thank you for the replies, i will need to think about this phenomena deeply although it appears clear to me that simple principles should suffice, unless you are choosing really sharp opening lines or playing at master level.
The OP's concerns seem not unfounded by any means, quite to the contrary. Indeed, many if not most beginning players seem nowadays to approach everything the wrong way around. Instead of studying endgames and tactics and playing OTB games with a lot of time, they (often quite mindlessly) memorize opening lines and play online blitz or bullet games by the dozens.
Basic opening principles + basic tactical skills will get you out of most openings alive, that's true.
But the nature of the opening makes a difference : if we take as an example the opening schemes you're suggesting, it's true that you won't expect a lot of fight in the first moves with these systems. You're delaying it for later. For me, the study of these openings is more about the middlegame plans and typical ideas you can implement there. And you still need some study, otherwise, you may just have no clue how to proceed (depends on your level of course, but let's say for an U1700 player).
OTOH, if you play some sharp, confrontational stuff (open games, sicilians), then you need to solve concrete problems early on, or you might just be lost very fast, even by applying sound opening principles. And this is a problem you'll meet at intermediate level (ie. ~1700-1800) rather than master level.
When it comes to opening choices, I think there are many valid approaches, and I wouldn't dismiss any of them without knowing the skills, ambitions and time available to the players.
Sharp stuff teaches you the values of every move and to stay alert, which is quite useful , while slow build-up develops your strategic sense and dispenses you of the need to learn too many lines by rote.
The main drawback of sytem play is actually that it's...systematic You have many basic recipes, which usually work well, but you may sometimes find it difficult to adapt when your opponent gets you out of your confort zone... Also you need to become pretty proficient at handling pawn structures. Just a different set of challenges
I'm with the OP 100% here. However, people want to read and study openings so that's the market. Goodness knows why anyone would want to study a Blitz game! BTW, did you realise that 98% of all games played on Chess.Com are Blitz (say less than 10 0 ) games? Slow Chess here is considered to be 30 0 - which incidently is the ECF's standard time for rapid-play! :-)
hicetnunc is correct in what he is saying but he is a much stronger player than most here. For most players, Blitz just entrenches bad habbits and detailed opening study is wasting time which could be way more profitably spent on other things if they want to improve their play.
In my Online Chess, players under 1600 regularly blunder away pieces (even their Queen!) to simple N forks, pins, etc.
Steve
Btw, I'd like to mention that for what I know of Robbie (his posts and information available on chess.com), I have the impression that his philosophy about openings suits him very well and he is making very intelligent opening choices.
I like pacifiques idea of studying with regard to typical plans and ideas and hicetnuncs of looking at pawn structures, these are much easier to remember than variations, I don't like opening theory, its boring and tedious, chess should be fun I think, chess learning especially, its not like learning Shakespeare! :)
Well nobody wants to learn about tactics from a blog post. Once you see the solution to a tactic, is pretty easy to understand the ideas behind it (i.e. you forked the King and Queen so you could capture the Queen).
When it comes to positional features, there’s a practical limit to what can be said. At some point you’re going to have so many bloggers talking about the same ideas that it’s going to be a lot of repeat. Same thing could also be said about other topics like mating nets, or basic principles.
However when it comes to the opening, there are so many lines and variations that each blogger can dedicate themselves to that variation. For instance, maybe you decide to start blogging and you notice that there are already strong players blogging about topics like positional strategy, tactics, endgame and different openings, but no one has blogged about the Hobbs gambit in the Bird’s opening, so you decide you can blog about that.
openings are tedious but i still think they are necessary for mastery because you can understand many concepts only as a hole.
@qrayons, how is learning about the Hobbs gambit in the Bird opening going to help someone, anyone for that matter, become a stronger chess player?
@TetsuoShima, with all due respect, learning openings in themselves is useless unless they are accompanied by middle and endgame plans. If you are a diamond member, please check out GM Sam Shanklands video on the Caro Kann for black, its probably the worst video series that I have ever watched, and I have no personal feelings of animosity against the Grandmaster for any reason, i am sure he is a fine human being. You could watch that video for a thousand years, a thousand times a day and never gain a single rating point because all it covers is opening variations. I even get the feeling that by the end of the series the grandmaster is thoroughly bored himself. His explanations consist of, 'its probably playable but i don't believe in it. . . .', 'black would probably be ok. . . .', what the heck is that? where is the fundamental understanding of the position and the long term strategic aims linked with the opening? I despair, I really do that people suck it up, so much so that i began to question my ability for rational thought.
I think most would agreee that it better to learn the Ideas behind a particular Opening than concrete variations of the opening themselves.
Though of course... if you are aware of a few traps in your chosen opening, they can be a source of a some extra points! :-)
Steve
The buyers want opening books. Whatever sells. Simple.
Most everyone collects a mass of opening books, before they wise up.
Many don't ever wise up. The forums testify to that.
Its so true, millions of unread opening books, i too have at least thirty i have not nor will ever read and they always include something like, the secret of the Sicilian unveiled, killer chess openings, winning with. . . , all nonsense, there is no secret to the Sicilian, there is no killer chess openings, all that exists is strong moves and weak moves.
@qrayons, how is learning about the Hobbs gambit in the Bird opening going to help someone, anyone for that matter, become a stronger chess player?
Your misunderstanding is in thinking that a blogger’s chief criteria in selecting a topic to write about is the level with which it will help someone improve their chess.
as a regular blogger i am amazed at the number of posts, mostly by strong players that are dedicated to a particular opening, look at the blogs section and you will see what i mean. Opening repertoire against the French or the Scandinavian, more blitz videos, Fantastic French, etc etc why should this be the case? Why do strong players think that weaker ones will become stronger by studying chess openings? You could watch blitz videos for a thousand years and never become a stronger chess player? Can someone please explain the phenomena?
Chess games are not won or lost in the opening, or should not be if one follows good opening principles, they are won or lost in the middle game and the end game, is it not the case? usually to some positional oversight or some tactical oversight, why then the focus on chess openings by titled players and especially those who profess to be teachers and coaches? or why the plethora of essentially useless blitz videos which teach the student nothing about how they should think during a chess game, can someone explain why these are not only the most popular but the most frequent.