If you don't learn a decent opening or some responses you will have an inherent disadvantage in the game. Seeing tactical patterns or knowing endgame techniques will be absolutely useless if you don't even MAKE it to the endgame. Here's what a bad opening can do for you:
Lose position, if you're can't bring out any of your pieces your screwed, especially if you can't stop an attack.
Lose material, this ones kind of obvious.
Lose control, if the opponent brings out a queen and you cannot bring anything out to stop it *cough cough Nelson* you won't make it to the endgame.
In short you can't make any patterns if you screw up your opening, or at least it'll be 10x more difficult.
One reason experienced players may seem preachy about this is... I think we all tried it. At least I know I have. I bought some opening books as a beginner. I spent a whole month memorizing lines from a book, taking notes, that sort of thing. Then I started playing, and sure some wins came easily when I was able to copy moves into a winning position, and that felt really good!
But my rating barely improved. Maybe 50 points.
Later I put the same amount of work into Soltis' book Pawn Structure Chess, and again into a tactics book, and again into Dvoretsky's Endgame Manual. Each of these 3 sessions (roughly 1-2 months each) gained me 100-200 points.
So yeah, as much as 600 rating points for the other stuff vs 50 points for openings.
So then when we (or at least I) see these topics online, I say, no, memorizing lines isn't that important. Do I memorize lines even today? Sure. Of course. But that's just the frosting on the cake. If you forced me to play totally different openings my rating would hardly drop 100 points.