The short answer is "it's English!"
I just explain that there are certain expressions that take certain prepositions, and that it makes no logical sense. There are just some things that need to be memorized.
Most (if not all) large transportation vehicles (e.g., trains, buses, planes and ships) take the preposition "on."
I also use a unit from Murphy's "Grammar in Use-Intermediate" that shows these preposition-transportation vehicle combinations.
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