[jhbadger]

In part because the US has created "pre-school", which children attend for a year or two before kindergarten. This is more like the traditional kindergarten that you describe -- it is intended to get kids to learn to get along with each other, play nicely, sing songs, etc.

[bombcar]

This is 100% it right here. When I was young, kindergarten was preschool (sometimes actually called that) and was also not mandatory. You could start school in first grade, and some schools didn't even have kindergarten.

Now preschool has standardized tests (!!!!!) and expected outcomes and kindergarten is worse. And 3K has been invented to be what preschool (and before that, kindergarten) once was. Of course, you can't go to 3K or 4K if you're not potty trained, which puts additional time crunches and restrictions on things.

Maybe it helps overall, but I'm not entirely sure it does.