521, you are either not a parent or you were one a very long time ago, and do not remember those first years. These are the years when they do the most learning and developing, and their ideas about themselves and the world are formed. Little ones who are active and learning at this age have a leg up for the remainder of their school years.
I know a couple of kids who attended "outdoor" preschools for 2-3 yrs prior to kindergarten. These "schools" were sure a lot of fun (lots of tree climbing, free art/free play, petting goats etc) but provided zero academics. When these kids entered kindergarten, they weren't familiar with structure and didn't even know their ABCs. As a result, they struggled a great deal, and unfortunately continue to.