What do you mean by a set-top? Are you meaning a DVR?
If that is the case then it is because there is just now technology that has the ability to link the wifi adapter directly to the display, prior to that you would have to use one of the dlna,miracast, etc protocals that not all devices support.
Then there is the issue of latency and bandwidth of sending something like blu-ray over wifi to the display.
While not having any cables is more convinent and looks beter, a cable will always be more reliable and more capabile then wireless.
das_tig - All those require an intermediary box and I am not aware of how to stream a live TV signal from my Comcast DVR to say a Roku.
Boosted1g - Yes, I am referring to a Comcast DVR or just plain digital set-top. I want to take the output (RF or HDMI) to a box that then transmits the signal over wi-fi to my Samsung Smart TV. That TV has Hulu, Netflix, etc apps which receive streaming video just fine. Protocols and latency don't really seem to be the problem.