Im not really sure what the best explanation is. But in the absence of strong evidence to the contrary, I’m going with “defenses have caught up to offenses” so that in the clear light of 2022 we can see that a lot of mediocre QBs put up grossly inflated numbers over the last couple years. And you’re right: Allen, Mahomes, maybe Burrow and Deshaun (ouch) are the only QBs pretty much guaranteed to give you a good show on any given Sunday.