You can’t buy a good webcam because the number of people willing to pay a lot of money for a high end webcam is very small.
At the lower end, people are satisfied with their built in laptop parts or a cheap webcam that sits on top of their monitor.
At the high end, people go down the rabbit hole of buying a do-everything mirrorless camera that they can use for so much more than just a webcam.
A high end webcam would have to be cheap enough that the first group doesn’t mind spending a bit more, but not so expensive that the enthusiast target audience just decides to buy a full-featured mirror less camera instead.
Granted, there is a lot of room for improvement in that budgetary middle ground, but how many people actually care? Common webcams actually perform decently when given proper lighting conditions. We’re not streaming high-bitrate 1080p H.265 on our 5-person Zoom calls. After compression and denoising the extra sharpness and low noise of a high end camera doesn’t add much benefit.
Enthusiasts are a difficult group to market to because they have extremely high expectations. They’d also rather spend weeks scouring the Internet for the perfect deal on a used mirrorless camera than to spend a dollar more than necessary to buy a high-end webcam.