My understanding was that buses don't have seatbelts for two reasons:
* Per capita and per mile, buses are far safer than personal automobiles.
* When buses do get in accidents, seatbelts are more of a hazard than a safety feature (e.g. by preventing impaired riders from fleeing the vehicle on their own).
The latter doesn't apply to personal vehicles to the same degree, since they're preventing fatalities in different types of crashes for the latter (e.g. being flung out of the front window).
Most vehicle accidents are between vehicles. Most vehicles weigh far less than busses. And most busses spend most time travelling at lower speeds on surface streets (though long-distance road coaches are exceptions).
I've been in a bus-automobile accident, at low speed (< 25 kph). The experience from within the bus was an abrupt stop, I've experienced similar from normal braking.
The auto involved suffered major rear-end damage. The bus's front bumper was somewhat scuffed.
High-speed rollover accidents, collisions with fixed structures adjacent to a high-speed roadway, or plunging from a bridge or cliff might benefit from additional restraints. Those circumstances are seldom encountered.