They meet lots of people, are very outgoing and can talk to anyone. They also have lots of friends their own age. Our homeschool support group goes on regular outings that mix adults and children of all ages, my older girl sings with the Vancouver Bach Children's Choir and takes classes in drawing and cartooning, the younger plays piano and is on a ringette team, and both are members of a children's circus troupe. They have time for all this contact with people who share their interests because they're not in school. Some people seem to take it for granted that this is a process that can only occur in school, possibly requiring a critical mass of children and the supervision of a trained professional in order to take place.
Socialization is the means by which people learn how to behave as adults in their society.
Taking virtually all the children and isolating them from adult society throughout their childhoods is just about the dumbest way of accomplishing this that anyone could have devised. Today's kids have plenty of opportunity to practice their social skills on other kids, but have an extremely limited opportunity to observe adults interacting normally. Many have only one or two parents and their teachers to observe at all. They are emphatically not welcome in most places where adults spend their time. If my kids are deprived because they don't spend all day among people acting out what they've learned from The Untouchables or Days of Our Lives, amen to that.