Some of those kids are lacking socially. It's not 100% by any means. The thing is that if you are home schooling your kids, you have to be intentional about socializing them in a way that you simply don't have to if you have them in a formal school setting. Obviously, some parents are going to fail at that but not all will.
Well the thing is if youre home schooling it's really easy to just never have your kid interact with someone they dont get along with. Thats the most important part of the socialization though. If you dont put effort into making your kids life a little uncomfortable Im not sure how theyll gain those skills.