The problem is these large companies need to keep these UI/UX experts on the payroll, and constantly refreshing their look is a big part of that. People also want systems that look fresh; in the same way that fashion trends change, or follow a large cycle, UI is frequently refreshed to keep a current aesthetic, because customers can and do care about aesthetically pleasing products.
Video games are a great example: better graphics have often come at the expense of story length and gameplay, but people still flock to current looking games, even when games with superior mechanics and inferior graphics are available. Mobile gaming is only an exception because the tradeoffs are too large for most people, and I have a feeling that will change as mobile hardware continues to do more with less.
As long as the usability tradeoff isn't too large for the majority of customers, they'll keep making that trade.
I'd say that there are some video game series that don't focus on the newest graphics, like Mario or Pokemon. The graphics have improved from their first incarnations, but I don't think either series goes for realism over all else. I wonder if there are any examples of UI that follow that same trend? I can't think of any at the moment, to be honest.