Except each role IS VASTLY DIFFERENT. Each role has its intricacies in not only build paths and champion pools, but in match-up knowledge, relevant objective control tendencies, overall playstyle, and different methods of laning pressure.
Asking Chris Bosh to practice his long range jump shot (the funnier one would be telling Shaq to practice his fuckin' free throws) isn't telling him to practice a completely different part of the game. Everyone in basketball should be capable of making a variety of offensive plays. That's the equivalent of being able to play a variety of different champion in League based on what you need to do. There are different champions with different ways of engaging and applying damage, or soaking damage and neutralization, that mean a champion who mains a role has to learn how to play a variety of both offensive and defensive picks that can provide unique situational advantages.
You never ask your Power Forwards to practice 3-point shooting because they should never be in the position to even have to make that shot as a remotely regular occurrence. Most probably wouldn't even try to shoot one as a joke in the middle of a game. And even then, it's preposterous to equivocate having a Power Forward shoot 3-pointers to making them play an entirely different position like you suggest we do in League constantly.
And no, no challenger level mid-laner could possibly hold their own in the jungle against other challenger level junglers unless they secondary jungle. They get shat on by challenger level junglers precisely because that jungler is THAT good at being a fuckin' jungler. You don't get to challenger by being so good at everything that you can stand at equal footing with every challenger player in other roles. Challengers outside of their mains roles are excessively inferior to Challengers in main roles. They might know how to jungle to a fundamental degree, but knowing only fundamentals is NOT how you get to that high tier of play.
And the reason you see players that CAN pull off almost everything in high school basketball is because the disparity is skill is much more apparent. High school basketball doesn't discriminate and make teams based on skill level, so the equivalent of a high school basketball prodigy will end up playing against good players and shit players alike. The NBA is effectively the pinnacle of competitive basketball in the United States, whereas high school basketball is just a myriad of different skill levels being shoved into the same team and playing against random assortments of other teams. Perpetual placement matches. It's not like some "Gold-level" hs basketball player is only going to play against, or even with, others of that skill level. He will play against the relative bronze-level scrubs because hs basketball doesn't actively discriminate and tell two teams that they can't play each other due to skill disparity. This means that even if he's the best Point Guard on his team, he just happens to be so good he can beat a more dedicate Power Forward in the same role because the Power Forward just happens to be inferior overall to him in every way.
League on the other hand, discriminates skills. League assorts random people of relatively equal skill levels only (or at least tries to) to play against each other. Someone in an off-role will simply always be worse.
The point of Ranked is to figure out how good players are. You don't figure out how good a person is at a game by making them play their worst. If we make everyone never get their main role forever, but rather a random assortment of all 4 other roles that they never play, and without consistency, the quality of play-making would be so bad that people would actively stop playing. If you don't queue up for a role, it's pretty fuckin obvious you're bad at it. You don't need to get forced into it just to have other people find out "Shit, he does suck at jungling, he should probably stay mid."
You put people at their best against people at their best, not the other way around. You are not competitive if you're not doing your absolute best. People play this game forever and begin maining a role PRECISELY because they feel like that's what they're best doing and best contributes to each team they play with.
Basketball players are measured and judged based on their performance in their given position. You don't judge a Center by the quality of their ability to play every part of basketball. You judge them by their ability to be Center, because that's what they do. In that similar vein, you won't judge a Carry main for his ability to play tanks. You judge him for his ability to play Carries. He of course KNOWS how to play every other part of League to SOME degree, but this is what he does BEST, and if you plan on using basketball as the backbone for your argument, then judging your ADC main's performance should be done the same way you'd treat your Point Guard's performance - based on how well they play THAT position, not how they play everything in the bloody game.