The driver has a seat belt. This is the foundation for why seat belts are mandated to begin with -- it is far easier to lose control of a car when you're unbelted, and that is a huge risk to everyone else on the road.

Beyond that, it's probably a matter of practicality that passengers aren't required to be belted in. It would significantly reduce the capacity of public transit. A better option might be to protect the driver from projectile humans. AFAICT that's exactly how modern buses are designed.

Also buses are generally much more massive than the vehicles they collide with, leading to much lower forces on bus passengers than on car passengers.

I don't know if this applies to general passenger buses or just school buses, but in the US they have closely spaced seats with energy-absorbing seat backs that limit the force on each passenger and how far the passenger can move in a collision.

Smaller school buses (under 10k pounds) are required to have lap and/or shoulder belts at all seating positions.

Long distance buses have seat belts in Europe.

And nobody uses them and it's not enforced.

Same in Japan and legally enforced.