The amount you have to screw up the results to get people to prefer the real results is greater than the difference in performance of the major engines.
Conclusion? Search quality is a cost center at google at this point in the game — they have no economic incentive to improve it further.
Or maybe they figured that most people do very simple searches nearly all the time.
So we know embedded ads all around the internet aren't challenging it in any way.
That said I don't know what percentage of their "google property" ad revenue is youtube, gmail, etc. Maybe they're cash cows?
I'm inclined to think it's still mostly search though simply because it's the most valuable source of lead. One where the consumer has buying intent.
Searching for a new car Vs trying to watch a show and having a car shown to you before it.