I've always wondered about that. Especially with school buses.
Maybe it has to do with bus material being different than car material. Or the amount of space in a bus versus a car?
On city buses, what really bothers me are those flimsy straps that strain my arms because I'm kinda short. If something happened, I'd totally get tossed! I can wrap my arms around a pole, but I can barely hang on to the straps when the bus stops or starts.
Maybe it's cause bus drivers have their special bus training and bus licenses. Or because they can't go too fast cause they stop all the time?
Still, it'd be nice during rush hour, when I'm not fully awake, to have some kind of precaution.
Or with Grayhound, on a long trip. Be nice to at least have the option to strap myself in befall I fall asleep.