Yes, scenarios where belts would save lives definitely exist.

However these city buses without seatbelts rarely have accidents, and even if they do they are fairly slow and they won't stop as abruptly as a car because they are so much more massive. So the number of accidents where it actually would have made a difference is very small.

And apparently this benefit is just not perceived as sufficient compared to the drawbacks:

• The initial cost

• The maintenance cost - people definitely damage such things in public spaces

• Them being in the way, slowing people down on busy routes, making people miss stops because they can't losen a possibly unfamiliar belt quickly enough

• The fact that 99% of people probably wouldn't use them anyway out of habit and because they have so little risk

• The aforementioned additional risk in some accident scenarios that further relativises their net benefit