Honestly, it's 90% of the problem. No doubt the crappy bitrates don't help, but it's mostly the camera. Even a fairly low bitrate 480p stream with a good camera looks great.
I've recently started using an old iPhone as a webcam and the differences are startling. Especially in bad light.
Though, it is frustrating seeing how much the bitrate ruins it. Comparing the 1080p output from the iPhone to what you see on zoom etc is depressing. But the camera alone can vary from abysmal to very good. Can't get excellent because of the bitrates though.
The providers (zoom, webex, MS) have lowered the resolution to lower their bandwidth costs.