Why does it ask for my wheel circumference on a direct drive trainer?

This may help you: How Does Zwift Determine My Speed?

And Zwift will only ask you for wheel size if you are using a speed sensor. If you are using a power meter or smart trainer Zwift will use power and not wheel speed to determine in game speed.

If you have aDirect Drive trainer make sure you are connecting it as a power source and controllable trainer.