I bought a cheapo capture card a while back but the quality is pretty terrible
Is there any reason PCs can’t just receive HDMI/displayport input from other devices through the same mechanism they output it?
For that matter also is there any reason HDMI can’t be run over Ethernet? Afaik ethernet has far more bandwitth than hdmi nowadays
It can, you have things called HDMI extenders but unless you know what you are doing they will not work as you expect.
HDMI 2.1 has 48Gb of bandwidth a cat5 cables maxes out at 1Gb, while a cat6 cable can do 10Gb up to 55 meters if installed correctly. Much lower then the 48Gb spec of HDMI 2.1
Last time I dealt with HDMI extenders (2.0 just announced; 18 Gb/s), you ganged 2 cat 6 runs to get it to work over any significant distance.