Why do we have to say “get on the bus”, but “get in the car”? It’s because we have to step up ONto the bus in order to get in, but not with the car. We just simply get INto the car.
There’s a historical story behind the phrase “get on the bus”. In the 19th Century, a voiture omnibus (a vehicle for all) – later shortened to merely ‘bus’ was a horse-drawn carriage to which you get on. As is with the modern bus, the voiture omnibus is significantly raised from the ground; therefore, one must get on in order to get in. Get it 😀
If a car is significantly raised from the ground, like a jeep or an SUV, the phrase “get on” would also applicable.
This is what a voiture omnibus looks like :