That's probably because balance in combat is easy to talk about, because it's easy to verify.
But all your follow-on assumptions are invalid: I'm not only talking about combat. I'm talking about the ability to solve any obstacle, any conflict, any problem just-plain-better than another PC.
Cheers, -- N
To a point, I agree with you, characters/players should have as many meaningful choices as possible--but I don't know if I agree with the underlying assumption implied by this statement, which is, "A game's rule system should allow all characters to have an equal opportunity at every encounter/problem/choice to have the same probability of success as any other character." Why have a DM at all if this is the case? Doesn't that change the game from an RPG to something else at that point? If the rules can completely govern "balance," you don't even need a referee (think: chess).
Well, if this is the case, why would anyone with this mindset play ANY pen and paper RPG with a
group? Why even have other characters, if your character (and conversely, every other character at your, and every other game table) is equally likely to be able to solve any encounter? If taken too far, this type of rules design ethos can fall into the trap of "everyone's special, so no one is." If everyone gets to "have the spotlight" on them all the time, then no one does.
Don't get me wrong, I think it's important for characters/players to have as many meaningful choices as possible. But one of those meaningful choices is to
choose to play a character that interests you, regardless of the "mechanical" consequences. And if this means the GM has to adjust encounters/play style to let that character have fun, well then that's why we have a GM in the first place.