You’re on the bus, in a seat, but if the bus driver finishes the day and left his hat behind, his hat is in the bus on a seat. Active/private/static vs passive/public/transitory. You’re generally in buildings but on vehicles, unless that vehicle is both private and enclosed. It’s not much more complicated than in[side] vs on [top of]; just keep in mind that it’s predicated on whether or not the encapsulatory nature of the object is necessary to its identity. For instance, you could also ride on a flat parade float without walls or roof, and putting a box on it to make it a bus doesn’t change that, so it remains ‘on’.
But why, exactly do we ride in a car but on a bus? Or sleep in the bed but on the couch?
“Get on the plane, sir.”
“Fuck you! I’m getting in the plane. Let Evil Knieval get on the plane.”
You’re on the bus, in a seat, but if the bus driver finishes the day and left his hat behind, his hat is in the bus on a seat. Active/private/static vs passive/public/transitory. You’re generally in buildings but on vehicles, unless that vehicle is both private and enclosed. It’s not much more complicated than in[side] vs on [top of]; just keep in mind that it’s predicated on whether or not the encapsulatory nature of the object is necessary to its identity. For instance, you could also ride on a flat parade float without walls or roof, and putting a box on it to make it a bus doesn’t change that, so it remains ‘on’.