He's... not. He really isn't. It sounds pretty and simple, but that's it. And as for the "why" we use that ruling, I feel it's more because to board one of these things is often a trip to begin with. It's a destination. As with a car/cab/limousine, they're more personal and doesn't take much effort to board... since you wouldn't often "board" one of these vehicles, seeing as it's more used for something that would use "on" when entered. And something that uses "on" all seem to also share the fact that they will accommodate a plentiful number of passengers or bigger loads over something that you would "get in". There's also when you would be boarding one of these vehicles, you have already gotten on something to board the vehicle, like a platform, a deck, a lift, a jetway, etc. Then there's the prep time for those vehicle before it can get going, the time for getting passengers on, procedures that need to be met, etcetera etcetera. It's always a miniature event when you think about it, and using on could represent how it's an event. And there's looking back to what getting "in" a car actually means. When you get in a car, you are already in your seat. But for a bus or a plane, you would first have to walk on what would be the bed of the vehicle to your seat, then sit in it.
Just a lot of little reasons for this.
"Cogito, ergo es eggo." A.K.A. "I think, therefore you are a waffle."