Why are beginners dissuaded from learning openings?

Telling beginners not to play openings is horrible advice. Of course you should learn openings so you get a good position and learn to play the game properly instead of just playing random moves.

The difference is that the significance of opening theory is diminished at lower levels. Some openings which are technically worse are harder to punish when you don't know why they are bad or if you simply blunder your pieces in the middle of the game. This is why you learn though, so you can punish poor play by the opponent.

It's important to learn other aspects of the game and not just focus on openings but you should also learn openings.

If anything, everyone should be dissuaded from playing fast games because whats the point of studying anything if we're gonna lose most of our games from leaving pieces and pawns unguarded or moving them to squares were they can be taken for nothing and missing all those simple tactics because of the clock.

Beginners are natural masters like capablanca, they play their own natural moves and in natural style, they don't copy anyone else's opening moves, they trust in their own ability and combinations they play in the opening come out of their own mind, not stolen and memorized from others , so all hail master beginners

Beginners usually aren't dissuaded from learning openings, they're dissuaded from studying theory. There is a difference, but it doesn't seem to be talked about much.
For example, I'd recommend learning the London, Slav, and Caro-Kann to beginners (a good repertoire because all the ideas are simple, solid, and similar while covering all the popular options: 1. e4 Caro, 1. d4/c4 Slav, London as White) BUT I would only recommend memorizing the first five moves, and then just playing chess from there, and I'm probably in the minority of players who would give even that advice.

Your time that you would spend learning memorizing theory is much better spent otherwise.
I'd recommend doing your three puzzles a day and the daily puzzle every day.
Play 5|5 if you want to play blitz and 10|15 if you want to play rapid.
Make sure that you never have more than a minute on the clock once you get to an endgame, and spend the rest of the time calculating in the opening and middle game.
Analyze and annotate every game first without the engine and then with an engine afterwards so you can see where you need to improve.
Improve your repertoire once you hit 900 and every 300 ELO afterwards, but stick with your current 'repertoire' until you hit 900. I'd recommend playing 1. e4 as White, and 1. d4 d5/1. e4 e5 as Black until 900, because you'll play many, many different types of chess games like this.

Beginners should learn openings to gain consistency but rely on opening principles to win more

For me, I found that at the beginner level, "opening theory" consists of a system, rather than a significant study. Beginners should really choose one opening and stick with it, and focus more on tactical opportunities rather than intense opening theory. However, opening principles are a must at any level.

Beginners should learn a small handful of openings, not "one" opening.  You need responses to 1. e4, 1. d4, 1. c4 as  black and you need to learn how to play an opening as White.

I think the thought process is that there is a difference between memorizing certain moves in an opening and actually learning it. The idea behind most popular opening systems is that it leads to a comfortable middlegame position where plans are well defined and the best tactics and strategies present themselves more directly (this can often even extend to favorable endgames, for the variations which have been explored in depth). So for most beginners they would be well advised to learn the basics of chess as after playing a few moves of a particular opening they will often find themselves lost and not knowing what the ideal plans should be in a given position. If they can first grasp the fundamentals and develop a sharper game sense, then they can more easily pick up on the finer points of opening theory.

in the words of riley freeman 'what is this? homework!?'

To understand most theoretical positions you need to be aware of a sequence of games ( and their analysis ) that led these positions, rejected alternatives etc.

Some positions are imbalanced, are difficult to evaluate and cannot be played intuitively, which is what a super GM may want as they want to throw positions to their opponent where they will mis-evaluate or mis-calculate.

On the other hand, where a GM can mis-evaluate and mis-calculate, an amateur will be totally lost.

The thing is that a beginner may not be able to play out correctly a pure static advantage, like the two bishops or a better pawn structure. Or may not be able to play well when they hold the initiative. Nevermind complex positions with material imbalances, which tbh I'm not sure at what level people play them well.

So even if theory does land a beginner somewhere with a tangible advantage, they won't be able to capitalise it.

So it's better to focus on calculation, middlegame and endgame instead of studying complex theoretical lines which they won't be able to play properly the moment book knowledge ends.

What does make sense however after some middlegame and endgame skills have been built is to e.g. study pawn structures and plans associated with each of them. Also don't play openings all over the place, keep some focus so that you end up knowing well how to play some pawn structures. 

It's only after that it makes sense to start looking at theory, which essentially will be a mixed bag of refined versions of the plans associated with the pawn structures you play and concrete analysis.

Actually, to learn openings you are better off getting Kasparov's predecessors and modern chess books.

You will learn a lot about the history of each opening, typical plans, plus Kasparov's views on each opening.

The annotations are very high quality, as is the games selection. You will be fully able to build a scaffold of a repertoire from there as you progress through the books.

The trouble with learning openings as a beginner is that you can't force the other player to move their pieces in the way you want, so you end up having to learn many variations of each opening, and beginners haven't developed the chess memory to remember all of that properly.

What I have found more useful is to learn common opening traps and how to avoid them. For example, learn how to defend and counterattack against Scholar's Mate attempts. And if you're black, and White plays the Queen's Gambit, don't take the apparently free pawn, instead you should defend your central pawn with another pawn.

GM Eduard Gufeld had many chess friends, and I was one of them.  In a game one day, after he had adroitly cancelled my "own" idea in a Classical French (an opening on which he wrote the book ), he shook my hand and tilted his head, saying "Mark, chess is Art, but chess is also Science!"  

If you want to build your own rocket, you would do better to learn math, physics, and engineering.  If you want to raise your chess playing level, you must understand opening tabiyas--the rules of chess grant limits to what the pawns and pieces can do.  Great minds have gone before us.  Study your openings!

If they prepare yes, but imo it's not productive i.e. to prepare long lines in the Yugoslav attack at beginner level.

Regarding pawn structures, it's important not to get bored ( what's the point if it makes a game boring... ) but any repertoire can be tweaked to control the number of structures you play.

E.g. at the backbone of my (W+B) repertoire are approx. 30-35 pawn structures ( without counting stuff that rarely shows, budapest, keres etc ). Some structures show with either color, so in that sense there's interoperability.

That's sufficient to keep me interested, but others may want more variety or less variety, or different structures to get different play.

It's a balance between realism/how much time one has vs variety. E.g. if I wanted more structures I could add secondary black defenses, e.g. adding an 1. ..e5 repertoire would add a ton of structures, as would adding the Najdorf.

If someone wants less structures, they can go for something minimalistic, taken at the extreme this would be like playing London, Scandinavian.

Of course, there is a whole spectrum of choices in between.

One needs to strike the balance that's right for them. Neither something like a system, e.g. London all the time is good, as it will get boring and won't teach much about chess, nor a repertoire of the size of a super GM, as it's just too large for non-professionals.

Even copying GMs who didn't adopt too many sharp lines is unrealistic, e.g. a full blown Karpov repertoire is quite unmanageable, nevermind when they did adopt a lot of sharp lines, as in e.g. a full blown Kasparov repertoire.

So something has to give, one mixes and matches, what is important is that whatever the choice is, it's studied/understood at depth and also it stays for a long time, otherwise no real experience is built around it.