Actually, that's not true. The original Kinect can be hooked up to a computer. People have already used it to control existing games, like World of Warcraft. The new Kinect that ships with the Xbox One does use a proprietary connection, but there is also a computer version being released, complete with official SDK.
That said, the game companies that innovate on this type of technology aren't all that interested in cross platform play, especially when it come to console games. The market is heavily single platform. The most you'd get from games like this is maybe Xbox, Windows, and PlayStation, and Microsoft is heavily pushing for platform level crossover between the Xbox and computer, so Xbox games would run on Windows pretty much by default (the Xbox One actually runs Windows 8).
Ideally, a game would be cross platform, but the reality is that most aren't, or are limited in what platforms are supported (and the quality of the port).