I'm
on the bus. <-- I usually hear this but it doesn't make sense. Why is it "on"?
I believe that you're thinking that it implies that
I am on the roof of the bus. If we say that I am on the car, it will also give the impression that I am
on the roof of the car.
According to an English usage book, we say '
on the bus' because we
get on the bus. On the other hand, we say that we are
in the car because we
get in/into the car.
However, strictly speaking, it is not wrong to say that we are
in the bus, but 'on the bus' is more commonly used. And I don't think many people will ask why; they just follow what most people say.
I hope my explanation clears your doubt.