Would you consider coffins?

Hahaha @filmfann

Prisms? I don’t get it. Aren’t they shaped like rectangular solids with rounded corners?

I thought that was the right way to say a rectangular 3D shape.

Well, this is the usual understanding of prism:

https://en.m.wikipedia.org/wiki/Prism_(optics)

But here is another definition, one I never encountered before, in geometry or anywhere else:
https://www.splashlearn.com/math-vocabulary/geometry/rectangular-prism

So I apologize for interrupting your thread. Always good to learn something, though.

Because they do not need to be. For planes, being aerodynamic is important, because drag quadruples at double the speed, and planes go fast. Buses are relatively slow, so drag is not really an issue for them.
Besides, giving buses a circular cross section will make them unnecessarily bigger, since all those arc sections at the sides, top and bottom, are wasted space.

Because to get to your seat, you have to be standing up. And the square-ish shape (from the front and rear) fits better with the width of traffic lanes.

Same shape as a commuter rail car. Seats, people, luggage space, rowdy passengers.
Not to be morbid, but it helps it stop rolling. A cigar shaped or Oscar Meyer Weiner shape would just keep rolling.

Planes designed to fly at high altitudes are pressurized so that the people inside will have enough oxygen pressure to survive. Pressurized containers are best to be cylindrical. Buses are never pressurized, so cylindrical shape is not necessary. In fact, rectangular (prismatic) shape for a bus is best, to allow good accessibility and generous storage space.

Aerodynamics are not important at the slow speeds of buses.

I agree with @kritiper: it’s simply the case that you have to be able to stand up and walk inside a bus. It’s a little room on wheels. Not so with passenger cars: you have to sit down to get in.