Why do they change cars after each Nascar race?

Answer 1

There are several reasons. First, time constraints. Teams have to move between tracks in a week and usually do not go back to their shops between tracks. It is not convenient for the teams to do so either. Also, drivers have little time to get to the next track and test a car - sometimes as little as 2 days. Making major changes to a vehicle isn't practical. Teams run many different cars which are tested and set up for the various tracks around the NASCAR circuit. It's simply easier to switch to another car than to make all of the necessary changes to a single car when going to the next track. Plus, they usually have a back up car in case of a crash as well. Tracks don't change as often as the cars do, so once you get one "dialed in", you keep it that way and build another for the next track.

Answer 2

they want the cars to be as good as can be