So wouldn't it make more sense for % max HP to be on AP champions?

In respect to the game's different systems of rock-paper-scissors.

Most of riot's % max has been put onto AD champions like exc

If you put it onto AP champions, which there are some with % max HP, but there are far fewer, then you'd have much better control of their limitations, but you could in theory create a champion with no significant amount of flat damage, and only % max HP, but only if they were AP, as AD implicitly has a substantial amount of flat damage associated with their auto attacks.

I mean, a good example of this in action is a champion like not AD, but AP. He has a reasonable amount of flat damage (noting massive 435 Q + E + ~700 damage ult before resistances), but then substantial % max HP damage, with his W (at 500 giving 15% per stack).

They'd be capable of killing a tank nearly as quickly as they could kill a squishy, but dial back their kill timer to slower than that of flat damage dealers against squishies, then you have a class of champions which are viably countered by squishies with their sustain recovery, but counters tanks. Where as when you start making champions like which deal with tanks, but also keep their implicit flat damage from their auto attacks, it seems like you're creating an even harder environment to balance the game because you can't reasonably remove their access to flat damage.