Well, two things.

First off, there's a place between "broken" and "high tier." Sure, Aquaman isn't broken, but he is too good. And of course he's too good: they gave one of the best pressure characters in Injustice an anti-zoning game.

Buffs (most of the time) don't alter the character's intrinsic playstyle, and that's what's really holding the low-tier back. Shazam's problem is not damage or combo-starters. The problem is that he can't start his vortex because he can't bridge the gap. Same thing with Sinestro: just what is the point of a great zoner when the top 5 happen to be complete characters?

The second problem is that a lot of low-tier characters win match-ups decisively. If you buff them, you now have to buff their natural prey, and then you're back to square one. People were screaming over the Kenshi/Kitana match-up in MK9, but kitana dominated a large portion of the cast. If you buff her, what happens to Sub? Baraka? This is precisely why the buff-only philosophy doesn't work.