You also need the interface to control the tuner (number pad, channel up/down, channel info, previous channel, etc.). Minor, but still something that needs to be implemented.

It is all feasible, but it's a cost add for something that a tiny number of people might want to use at all. Cable and satellite owners don't need it. TV owners also don't need it unless the HDTV is fairly old and they haven't already got an OTA tuner. Really the only folks who would need it are people with systems that have a front projector but are relying on over-the-air HD and have not alread bought an OTA tuner. How many people fit that category? Most projector owners already addressed source availability before they got the projector, through either cable service, satellite service, or an over-the-air tuner.