Why can’t Genymotion use the camera on my device?

You can use a camera, but the supported method depends on your product and setup. Genymotion Desktop can use your host webcam, and OBS Virtual Camera works only under certain conditions.

If you use Camera and Media Injection, your physical webcam can be granted access in the browser and assigned as the device camera. If you meant OBS Virtual Camera, it is supported in Genymotion Desktop 3.10.0+ and requires 720p output, but it does not work with Genymotion Desktop on Windows and is not available with Genymotion PaaS at this time. See How to use OBS Virtual Camera with Genymotion and Camera and Media Injection.