I guess it comes down to trusting that they'll sit on that big stack of user data, exactly the same stuff Google used to build a trillion-dollar company, and continue to decide day after day not to sell it. Will they continue to hold out if VCs get involved?
Yes, I'm aware that all the same arguments apply to DuckDuckGo, and that Google & Bing already explicitly do sell this stuff, but the stronger the promises, the more I demand to see proof.