Why can't put a camera in the middle of screen and use Eye Tracking on Vision Pro?

The Vision Pro already has a great amount of cameras for eye tracking. You shouldn’t need any modifications to use it. From Apple:

“A high‑performance eye‑tracking system of LEDs and infrared cameras projects invisible light patterns onto each eye. This advanced system provides ultraprecise input without your needing to hold any controllers, so you can accurately select elements just by looking at them.”

these forums are user helping other users like yourself, it's sort of going to a

physical apple store and asking other customers their advice

you can use their feedback channel to pass them suggestions

Feedback - Apple Vision Pro - Apple