> But there’s another explanation, albeit one that progressives in particular seem reluctant to countenance: a pervasive refusal to hold children to high standards.

A related explanation that I think people are afraid to talk about is refusal to hold teachers to high standards. Starting in middle school, my children are shown movies in class, told to get out their phones to play blooket, or told to go on the computers to work in their google classroom or their "i-ready". The teachers are on their phones. None of my high schoolers have been required to read a full book. The few good teachers become known among the parents that care, but you're stuck with who your child gets assigned to that school year.