They're different because the funnels are different. If Bing were to eviscerate Google Search overnight, YouTube and AdSense (other companies showing Google ads on their pages) will still continue to draw in the clicks and the eyeballs.
No. If Bing eviscerated Google Search, then MSFT could profitably serve and sell ads on video content, so MSFT/Bing would immediately want to set up a video site, and lure content creators off YouTube.