>I ask again, what is so impossible about just rendering the 60fps, and then not rendering any more?

There seem to be two ways to interpret your question:

>1.) Why can't games render 60 frames per second always?

Because some scenes are more complex than others. Rendering a complex scene can longer than 16.7 ms, and there is no way around that.

>2.) If a game comes far below the frame completion deadline (e.g., completes a frame in 0.5 ms, where the deadline is 16.7 ms), why doesn't it simply stop doing anything until the deadline has passed?

I do not know the answer. But I can say that many games actually do this. It is usually referred to as a frame cap.