The reason I've heard is that the macbook display assembly simply doesn't have the depth to house a decent camera. The camera bump in phones exists for a reason - and phones are already way fatter than macbook displays.
I have no idea if thats true though.
Also this doesn't answer the question of why USB webcams are so terrible. They have tons of room.
I could imagine a bump towards the front that would sink into the body when closed, but doing it in a way that looks and feels good also seems rather hard - but also harder to avoid should they want to implement things such as face id.
Honestly though, I think the real reason webcams on laptops suck is that until recently 95% of people didn't use them 99% of the time.
Also Apple: We can't give you a better camera because we gave you the thinnest laptop evar!
"We think you're gonna love it!"
I think apple deserves criticism for a lot of things they do, but their battery life is fantastic. (Well, unless you've got half a dozen electron apps running. But thats not apple's problem to solve.)
A good lens is more difficult to fit in the Z-dimension but that doesn't constrain the resolution of the sensor. A small bump in the bezel and a corresponding recess in the base would allow a better design of lens.