I found this video:
Hack #3 [3:53] seems pretty interesting... So apparently the reason why I can't use more than 2 USB extension cables on my USB webcam is because USB cable standards are crap, and the signal is pretty much guaranteed to be degraded after 3 meters. However, Cat5e cable standards are strict, and signals can be reliable up to 100 meters.
So if I wanted to use the half a dozen USB webcams I have lying around as security cameras, without having to have a small laptop/RaspPi connected to each one, theoretically all I'd have to do is solder a couple usb-to-ethernet connectors, which literally just solders the 4 usb wires to 4 of the Cat5e wires, and then back to USB on the other end, and in theory, I should be able to use my webcam up to 100m away from my PC (all I really need is 20m though).
Has anyone ever tried this? I know that you can buy usb-over-ethernet repeaters that also have signal filtering chips on them, and some are even active repeaters that also amplify the signal, but is any of that really necessary to transmit the USB signal 20 meters over Cat5e?