It's really not, the human brain is _really_ good at abstracting.
In pretty much exactly the same way you don't have to pay attention to balancing to walk, your brain learns to do the same thing for cycling and driving. It abstracts the mechanics of the car (e.g. turning the wheel to turn left/right) and sort of extends its view of your "body" to the car.
This isn't an instantaneous process of course, it takes time to build up that abstraction, but it absolutely exists. If your brain wasn't good at doing those types of abstractions then walking/running/typing/speaking/breathing/etc. would take a phenomenal amount of attention and practice.
