actually, i strongly believe that the most benefits come from how (not how much) parents interact with and care for their children. i went to kindergarten too, and i grew up spending a lot of tome outside with other kids with plenty of opportunity to socialize, both supervised and unsupervised, but lack of my parents availability (to put it mildly) created an anxiety that completely ruined any social ability that i would have been able to acquire otherwise.

so sure, each case is different, and you are not wrong, but you are putting to much value on good institutions. the heavy lifting still comes from the parents. your children are doing well because you care that they do.