why do we make a distinction

If you study English as a second language, this is probably one of those things you were told just to memorize: you say “get in” and “get out of” for a car, taxi, etc. but you say “get on” or “get off” for a bus or train.
But why do we make a distinction? For things like horses and motorcycles, “get on” makes sense, since we are literally on top of it. However, we are physically inside a bus, so why can’t we say “get in a bus?” The rule is:
If you can walk onto it and stand up inside, you say “get on”. If not, you say “get in”
To illustrate, let’s list out some other methods of transportation:
➡️ Get in a car
➡️ Get in a taxi
➡️ Get in a truck
➡️ Get in a van

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