The benefit of going to school for kids in the US at least is that you get to move a year closer to getting out.
A cost is that if you don't go, you get held back (and get the according social stigma).
Another poster below asked
> But why did Japan's snap first?
I think one of the reasons here is perhaps that kids automatically advance through the public schools system for the first nine grades regardless of whether they actually attend school or not. So people can stay even with their friend groups.
I've known/worked with a few kids who didn't go to school, and while some of them had personal issues of some kind or another, others simply just didn't see any cost to not going to school and hung out with their friends when their friends were out of school and otherwise enjoyed life.
Another difference, I think, is that with bullying in US schools, you can usually escape it to a certain extent through different classes/activities, but in Japanese schools, you tend to stay with your homeroom class for every subject, so there's no chance to get away/make a different friend group except for club activities and the next academic year.