There are quite high G-forces acting one the drivers when they accelerate, break or turn corners. You need a certain amount of fitness to be able to deal with those.

You also obviously need good reaction times.

Basically you need to be able to play a video game perfectly while strapped into an extreme roller-coaster.

It might not seem like much, because they all use cars that are similarly good and are all the top drivers that are available.

However if you have ever seen a normal person try to drive anything even close to a racing car around a track you will see that this is not as easy as it might seem.