First of all, who says that a Bar Mitzvah is a celebration for the boy and not for the parents?
Imma613 – In most yeshivas in Lakewood and Boro Park the boys do not come to the seudah. There are a number of reasons. First, the boys end up staying late and don’t function well in class the next day. When you have the entire class not functioning it doesn’t work. Try then to imagine 2-3 bar mitzvahs a week.
Second, the boys can and do get rowdy, and sometimes turn a nice Yiddishe Simcha into a wild birthday party.