It's obvious that TV makers have decided that seeing artifacts is not preferable. I wish that was toggleable.
I claim it's because DRM, since if it wasn't for DRM wanting control of the whole reception to display path to be protected, you could just stick an open computing device in between the streaming signal and the display device, and make your own technical choices.
You still can, AFAIK, for content that doesn't require HDCP.
It's because we have a trendy now-decades-long wave of product managers and designers who assume their job is to know better than the user.
Open source software is not immune.
cough systemd cough
Cable: no, you'd need a CableCard, and approved device (i.e. DVR), etc.
Do you mean Unix-like? I'm not sure what you mean by "open" in this instance, but Unix definitely was not very open, leading to nascent platforms and movements such as GNU and Free/Libre software.
I recall reading about TVs with multiple tuners to resolve adjacent channels when flipping, but I don't remember if they actually made it to market.