You can say 'get on a car' but it doesn't mean the same thing! It means to be on top of a car - for example standing on the roof! The point is that you can't stand up in a car or taxi so you have to get in it! When you get ON a plane, bus or train, you are still standing, and you walk until you 'get IN your seat' on the bus / train/plane, or 'take your seat IN the bus/train/plane' A bike is different - there is no interior so you can't get IN it, you have to get ON it. Hope this helps to explain the difference.

The thing is we sit in the bus or in train. but we sit on bike that's why it is so. (about train and plane I am not sure)

You can say get on a bus or get on a train. It describes the action of actually stepping from the platform into the train or the pavement onto the bus. You might say catch a bus or catch a train. But you wouldn't say catch a plane or catch a bike. I caught a plane to London I got on a plane to London, but we waited hours for take off. I rode my bike home I took a bus home/I got a bus home. I took the train home/I got the train home I got on my bike and cycled home. I got on the bus and went home. I got on the train and went home. I caught the train home. I caught the bus home. Hope this helps!

Actually, you can say "get on" for buses and trains as well. For example: "Please get on the bus." "We got on the train."

谁说的？可以啊！