Seat Belts Save Lives

I know you have heard it. So have I, but I've often wondered. If seat belts save lives, why don't school buses have them? Well I did some searching and found an article on www.nhtsa.dot.gov that essentially says that school buses are made safe enough that seat belts are not necessary. This begs the question, why don't we all just drive school buses? Or, why aren't cars built as safe as school buses?