I posit the real reason is because no-one makes a decent webcam for desktops and laptops, not even Apple.
Apple uses a lower-quality webcam in laptops because of the thinness of the lid.
Logitech needs to stop using cheap plastic lenses and excessively compressing video. USB3 has sufficient bandwidth.
Also it's just compressing to MJPEG, which is far higher quality than any subsequent h.264 compression your CPU would ever do. The Logitech Brio transmits raw data over USB3 like you're asking, but the resulting image quality is identical -- I've tested them side-by-side.
(Ancient 920's performed on-camera h.264 compression which was later removed -- if you have one of those, be sure to select the MJPEG stream instead.)
(HN discussion at the time: https://news.ycombinator.com/item?id=25869460.)
You absolutely do need to manually set and lock exposure, focus, white balance, and ensure you have proper lighting. Once you do that, it works like a charm. And this advice is what pretty much every streaming tutorial on YouTube advises.
The softness issue is particular to the Logitech Brio, which he notes, and which matches my own testing. The C920 doesn't suffer from that.
Yes I wish the software for the camera settings was more intelligent. But ultimately that's a software issue that's fixable with manual settings. The hardware is solid, provided normal lighting conditions.
The bandwidth needed for a high quality 1080p stream isn’t gonna be much more than one from a shitty 1080p camera. The better image would benefit from less compression / more bits, but it’s not an orders of magnitude difference.