I think for the most part it's due to scrub mentalities. As much of a cliche as it is, yes, I sincerely think that's the case. Even those that at some point weren't scrubs that have thrown in the towel, their reasons too are more often than not the same kind of pitiful excuses and laziness as the worst of the scrubs that have thought all their heavily armored CAS clad in over-satured red and black armor were works of art and quit even exploring the character editors possibilities due to laziness("I'm already at level 99, and without any more new pieces to work with this game is dead to me.")
Then what's worse is everything that should be telling these people how there is an issue with their mentality is just more proof to them that their beliefs are correct. Oh, Omega won a tourney with Nightmare so he's OP. Viola is relatively safe, she's unbeatable. A few great players use Raphael, he's broken. It's...I don't know. It seems like a bizarre mixture of selfishness, confirmation bias, laziness, and paranoia. And then what's even worse is if we try to explain that the greatest players are so great because they've worked their asses off to achieve that level of skill, we're all a bunch of elitist pricks.
I don't think there's any way this can be fixed, I think it's more a thing of human nature and the kind of circumstances required for this(e-sports if you'd prefer that term) that aren't the same as other competition. In other sports, things end up often starting with the players being much less privileged and having much less to do with their free time(as well as less free time fairly often). Play in the dirt, or don't play at all. But with this we're talking about a format that requires money and time to even give a chance to: $200 for a console and another $60 for a game. You don't need that kind of money to get started in amateur basketball or soccer, you can just play most anywhere with a hoop or goal for free if you provide the ball and players. And people that can afford spending that much have plenty of other options available as well, many of which require less effort for a more immediate feeling of reward(even when the "reward" is only self-gratification, many games offer this such as Call of Duty). Success in pulling people away from desire for gratification to actual, meaningful achievement is...well, I think the odds are grimly in favor of failure.
Then there's the other fun fact others have already brought up being that lots of people play games, but it's an incredibly slim minority that actually compete and an even smaller percentage of that group which are genuinely proficient, let alone masters of their game. It's not exclusively the amount of effort put in, it's also the amount of time and the quality of the effort. How about an example? A player with a desire to compete as Yoshimitsu may practice performing one of his numerous just frame attacks until they can perform it a majority of the time. A player attempting to become proficient will study the move to find more uses for it, things it can evade, and how it relates to spacing. A player attempting to master Yoshimitsu will study the move, how it can be mixed up with other moves, which attacks it can and cannot defeat, how it can be used in combos, what movement it causes, what it does to the opponent in all possible situations, and most importantly how it can be used in a practical manner. We're talking a shitton more work, effort, and knowledge here. And it can be achieved in a comparable amount of time, depending on individuals and their cognitive abilities.
But that sets us right back to the claim that the scrubbiest of players make: better players are all elitist snobs that hate everyone else. What can we do about this? Nothing, I think. These weakest individuals will see everything as more proof that their beliefs are correct. Want to play with people that give you a challenge instead of wiping the floor with lazy, incompetent players? You're being elitist. Want to be nice and try to help them up their game a bit? You're just doing it to stroke your ego. And it just goes on like that. Everything is proof their beliefs are correct in their minds and there is simply no reliable way to change this. Teaching them how to punish Nightmare isn't going to make them believe Nightmare isn't a gamebreaker. Trying to gently, subtly coax them into learning how to not fall for RO traps won't teach them that ROs are a fair mechanic not worth bitching about. The changes necessary to turn a scrub into a competitive player(let alone a proficient or truly masterful player) must come from within and cannot be forced by external influences. I think that's why all of this happens.
But as long as we keep on trying and try to avoid the same kind of self-perpetuating systems of bad beliefs scrubs use, I think we still have hope. Hope of bettering ourselves.