Actually they do question it. And here's an answer from Quora.com:
The general rule:
For vehicles, anything considered to have a large floor space is ON and a small floor space is IN.
The reason:
For walled vehicles, large floor spaces are considered platforms, small floors are considered containers. “In” and “on” for physical space is all about distinguishing containers from platforms.
Therefore it's in a car, in a small boat, in a helicopter, in a rocket, in a hot air balloon, in a small plane (think of a an old biplane). A lift would be in.
But we say on a train, on a ship, on the space shuttle, etc.
When the platform has no walls (i.e. it is not an enclosure) we say on, too. So it's on a stage, on a table.
Platforms needn’t be large, they just need to be platforms instead of containers. Think:
On a horse, on a bike, on a skateboard. Our feet are like platforms for our bodies, and we say to travel on foot. You can put your foot in a shoe.
Because the language is dynamic, and because it is old, some exceptions will emerge but this is the basic rule. If you stick to the general rule you'll rarely be wrong. Once you begin to visualize objects as platforms or surfaces, on becomes logical too for other kinds of things: on a screen, on paper, on a page, and so on.
Visualize this:
In a car, on the car roof.
On a chair (no walls), in an armchair (three walls but note some people say on because it's a chair), on a sofa (larger surface).
And think:
In a book, on a page. A book is a container of pages.