Because user experience. Simply selling a screen that can move like this one with the proper touch inputs is going to require multiple drivers to make it behave correctly. Selling the screen by itself and now you have this issue of supporting all sorts of hardware (and you just KNOW someone is going to buy it and try using it with their Windows XP machine).

Tying it to the computer allows them to control the entire user experience including adding specialized software to take advantage of such an interesting screen. There is a reason Apple owns the full stack and why Microsoft and Google are trying to get into that space: it offers the best UX and ROI.