The point of school is for students to learn and students learn best when they are not being asked to concentrate past their breaking points and they have downtime to digest and integrate what they've learned. American public schools are not designed this way (neither are most private schools)
Curriculums tend to accrete over time, not only because of the natural kludge-cruft cycle but because the world is genuinely getting more complex every day and there's more stuff that's important every day.
So that's a big pressure to cram ever more into less time, and no one wants to give up any of the old core curriculum stuff for good reasons, stupid fiefdom reasons, and stupid conservative politics reasons.
Not a great recipe for good educational outcomes.
