A car window is possible to open, which means that it "hangs" in a contraption in the door that disconnects it from the car a bit. And because the window would break eventually if it wasn't "rubbered up" a bit, it's also protected from vibrations.

A bus window, on the other hand, is an important part of the side of the bus in the regard that the bus needs at least a few windows to be intact, otherwise the roof will not be properly supported. In that kind of construction, you WILL feel the vibrations more, because you are literally leaning against the side of the bus when you lean against the window.