I think the factor noted above, of parents pushing in high school and no longer there to do it, is certainly one factor.
I do think high school prep is another. My daughter’s high school (a public, non grade inflation one) is “notorious” for its graduates finding college easier than school - and this includes those going on to ivies and other T20 schools. Certainly, my daughter who is at a T30 is finding this true as well. She’s performing better grades-wise with less work than in high school.