Many buses do in fact have seatbelts, generally tour buses or long-distance buses (so not the regular type, low-floor type of bus that stops frequently at bus stops).
Although you may have been implying that in the question – and asking why, when you are in a bus with seatbelts, it is voluntary to wear them. A possible reason is because it means that buses can’t carry more people than they have seats, as those people who are standing wouldn’t be wearing a seatbelt (so crowded city buses with standing passengers wouldn’t be allowed).
scubydoo – this depends on the jurisdiction. I know there are some places that don’t require seatbelts in the back seats (I think Italy is like this, for example). However, even in places that do require all occupants to wear seatbelts, they often don’t require people in buses to wear seatbelts.