Because one of the biggest ways that child abuse is detected and abated is by interaction between students and children. Isolating children in insular communities is a really great way to abuse them and hide the evidence of it.

Regular interaction with non-custodial adults provides more opportunities for child abuse to be uncovered and stopped. You can go on youtube and hear myriad testimonials from former homeschooled kids that were abused for years on end. And nobody knew. Nobody even had a chance to know.