I don't think Yahoo actually wants to be a search engine. I think they just want people to look at their ads.
Yup, which is why they've licensed someone else's tech to power the searches for most of the company's history.
By partnering with a browser: they can run searches through Google's servers but strip the Google Adword adds and replace them with Yahoo Ads.
Wait, what? You think Yahoo is going to use Google to power their search engine, without paying them? And you think Google's lawyers (let alone their technical team) would really let that fly?