You don't have to take the SATs unless you're trying for college. You're not necessarily seeing reading and math skills drop. You're seeing more kids who wouldn't have taken the SATs doing so. You're sampling from a more complete population, not just the top 75%.
(And yeah I'm fully ignoring the postCOVID brain drain. That's going to be a shadow in test scores for years.)
And the new Toxic Avenger