Comment by axiolite

> A niche thing, but it shouldn't be expensive to implement, and the ports are already here (usb-c, hdmi).

A Display Controller Board for driving a laptop screen from an external input is going to add at least $20 to the price of the laptop. Considerably more expensive than the $6 USB-HDMI capture devices which do the job (and have more utility).

The HDMI output port on your laptop (if you even have one, many only offer miniDP) can't just be run in reverse. The board would need to be updated to allow switching it between output and input, at considerable extra cost.