First, historically mass education of the type we now employ is only about a century old, and designed to produce factory workers not to promote creativity. Learning to handle monotonous boredom and obey orders was of primary importance to a factory worker.

Second, much of this is unprecedented. When my parents were children, they didn't attend Kindergarten, and first grade was a half day. When my parents younger siblings were children, Kindergarten was a half day. When I was a child, I didn't attend preschool.

In 3 generations we've added more than 2 years of education for every child, and that education is qualitatively quite different, even from what I experienced (Kindergarten 23 years ago)

When I was in kindergarten the majority of the day was playtime, we had 30 minutes of recess, 45 minutes of mostly freeform PE, 30 minutes of naptime, and a few hours of structured playtime (drawing, making crafts etc...).

From friends of mine who are teachers, since no child left behind, all of that has been removed or severely restricted. Recess is now 15 minutes, PE is more structured and not every day, no more naptime, and they don't have time for freeform activities because the curriculum is so dense.

In Kindergarten we spent almost the whole year learning the Alphabet. Now the curriculum assumes that children already know it.

Teachers know this doesn't work, but schools are obsessed with testing and metrics to the determinant of real education.