Why "step into a car" but "step onto a plane"

Why do we say stepped into a car with cars but can't say the same with planes? Instead we say stepped onto a plane.

Solution 1:

Possibly because aviation language has a certain amount in common with maritime language, and we get onto a boat.

Solution 2:

Scale often matters. One steps into a canoe or rowboat but onto a yacht or an ocean liner, into a car or van but onto a bus or train. Most of the aircraft most people board are large commercial airliners, not small private planes.

Solution 3:

Mentioned this elsewhere, but it seems the controlling idea is whether the user normally sits or stands. If the user is "in" the boat, it is a small boat and the user is typically sitting. If "on" the boat, users are comfortable standing. The same applies to planes, buses,trains, automobiles, and elevators. For things that are straddled - fences, horses, bicycles, farm tractors - "on" is used.