But I have wondered why one of these companies with billions of dollars to burn hasn't tried to create something new as a strategic initiative.

They have; Taligent comes to mind. You may not have heard of that -- or more likely, you have but just forgot about it -- but it's a good object lesson (no pun intended) in why a successful new OS is hard to just conjure into existence. There has to be a crying, desperate need for it, not just a vague sense that This Time We'll Get It Right.

You could probably cite OS/2 Warp as a less-obscure example of the same phenomenon.