Many, many reasons.
I'll post what you want me to post first: we have failing schools. We have an educational system too invested in standardized tests as young as third grade. We have some teachers who are horrid. Horrid teachers are terrible for learning, one of the key factors.
But then
We have a stupid school calendar focused on allowing working parents to work rather than on what's best for kids.
We are failing as a society, and yet expect children to come to school "ready to learn" (this one cannot be overstated).
We expect schools to do everything now, so they operate as day-orphanages rather than as academic institutions. For instance, just consider the modern idea that "behavior is communication". So a teacher has 150 kids a day at secondary, a kid is messing around trying to get friends to laugh--normal teenage stuff--and teacher is told they must investigate behavior bc kid is displaying an "unmet need".
This is unsustainable, of course. No teacher can "investigate the unmet needs" of 150 kids they see every day. And grade their papers. And contact parents. And, and, and.
How much more you want?
This doesn't even factor in the, say, shooter trainings we just went through again last week, wherein I have to explain to five year olds how to escape the building should a bad person with a gun come in.