Why is it that in many racing games, the car being viewed spins around its axis while the player is idle?

I can think of a few reasons:
- idle animations are a long-standing features of many video games, and can be coded to give more personality to characters or the game itself, add to the realism and immersion, or just a fun Easter egg put in by developers. They have been around since at least 1978
- in the case of racing games, it allows the model and design of the car to be showcased more fully, where usually only the rear would be visible during gameplay
- it helps prevent screen burn-in, which was more of an issue with older style TVs
- since racing games that came before had such an animation, it is perhaps just an expected trope that developers feel compelled to include

https://en.wikipedia.org/wiki/Idle_animation

To add to patrickk's answer, often at car shows cars are displayed on a turntable. My guess is that they may be trying to recreate this "car show" type experience...

https://youtu.be/XidUFxxJdSI?si=NzWcv6Azrj6X98UC