Many private schools make it very clear to the parents, as well as the kids, that there are plenty of other students that want the slot the child has, which gives them leverage to educate how they want.
Public schools, at least in California, are beholden to the loudest, worst-behaved parents. There was a serious disciplinary issue recently at a local school that resulted in a multi-day suspension, which pretty much everybody involved agreed probably wasn't going to change the behavior. After digging a bit and having some frank conversations, the administration wouldn't pursue any other form of consequence because of fear of being sued by the parents. If expulsion were a legitimate option it would have given the administration bargaining power, but for anything which defending an expulsion is not a slam-dunk win in court, the only 3 "safe" options for teachers and administrators are:
1. Loss of playtime during recess/lunch
2. Detention
3. Suspension
I don't know how big of an actual problem torts are, but I see a lot of bad policies (in education and elsewhere) enacted due to fear of torts.