If HYPSM reject, say, 65% of students w/ perfect GPAs and SAT/ACT scores, that means 35% get in. That's good odds, no?

I do see the problem but facts are facts. But, to your point, the rejection rates of perfect scorers might increase if perfect scores are increasingly achieved through multiple attempts, to the point that perfect scores will no longer have any predictive effect. At the same time, universities could also impose limits (i.e., they will consider scores from the first 2 sittings only, will no longer superscore) to stem abuse.