Haha funny question ! I think it is to preserve the secret about where Hogwart is. Would you take all the parents by train ? I guess, with the points system, they learn about fairplay, good attitude, stuff like that. Punishments are big enough, don't you think ? And in Harry Potter, from what i felt, they have a big respect for the teachers and the teaching, so there's no need to call the parents to make them understand something went wrong.