I think it would be an overkill to use a networking protocol to connect exactly two devices. Plus if you have something specific to video stream transfer you could maybe do some optimization specific to that use case, although I can't think of any at the moment.
The parent comment was specifically wondering about HDMI, the other examples are given saying that those have some reason to exist, while he didn't see any reason for having HDMI instead of recycling some other protocol.
