Most PCs sold aren't sold for gaming, and having the PC standardized to the point where every PC is game-ready requires game-related hardware to become so cheap and so sophisticated that the latest $400 graphics boards don't offer a noticeable improvement over integrated video. There's also the simple fact that most people don't connect their PCs to their TVs and likely don't keep their PCs in their living rooms to begin with.
This was never meant as an advocacy thread. It's fun to speculate about the future. The timeframe, I guess, would be like seven, eight years from now, at the end of the current gen--or a couple iterations of Windows. Will we still have graphic cards then, will we be plugging GPUs into the motherboard? I expect they will have become more standardized by then, as it'd otherwise be hard to utilize them for tasks other than 3D graphics.
I'm assuming that high-speed wireless network will be fairly common and that there is a box to stream the audio-visual data into. From the computer a game would need: certain number of CPU cores, GPU cores, RAM, access to storage, and networking. The problems with basic usability are solvable in my view.
Finally, there's marketing. There are too many PC vendors to communicate a unified vision to the market of the PC as ultimate gaming platform in order to have a large-scale hardware launch, and the ones that are big enough aren't dependent on gaming for their core business.
That's a fair point, but the PC are pretty much indispensable. People will just buy them. Gaming is not the main selling point now, but it's a high margin area. We'll see greater effort in the future.