The better question is why we say get "on" any of those others. We move from the outside of a car, plane, train, bus to the inside. We get in the bus, we get in the car.
If we got a ladder and climbed up and got on the roof of the train, then we would (logically) say we got "on" the train.
Now, trains and buses are bigger than cars and it is understood that "on the train" does not mean you climb on top.
But if you said you got "on the car" we would think you climbed on top of the car. Or sat on the hood.
That's the way the language is used.
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