At least in the recent past Americans had large families and civic organizations like churches to expose children to normal social structures. Today we are losing even those, and age-segregated schooling is the dominant mode of socialization for children.
Ah, right. The way they are designed.
Like you say though, there's no real alternative. I wish there was something else with widespread adoption.
Lots to think about here.
Also, speaking delicately, a lot of poorly-socialized homeschooled children have poorly-socialized parents, and were doomed from the start. I am gregarious and involved in many different social circles (athletics, music, engineering, etc.) and should be able to expose my kids pretty thoroughly to them.
Home schooling isn’t for everyone and not everyone has the means (lots of ferrying around kids between activities as well as the time needed to educate, basically it’s a unpaid full time job and many families require those hours spent generating an income to survive) but the image of home schooled kids being unsocialised is pretty much a myth.