Links have anchors (link text) - it relates target page with a query.

I think the catch is with cost - it's very easy to abuse click-factor, and the only way to fight that is to track and filter very aggressively and still not be sure how much you were 'gamed'. Link-factor is much costlier - you have to buy a domain, a server, get your own site promoted first, before you can signal anything. Link farms that don't have trusted in-links are worthless, no matter how big.

In general, 'user data' might be costly and thus higher quality, or cheap - noise, so whether it's worth buying depends on that. Anonymous free click streams are noise. If we get to scale with an anonymous click-stream paid for with LN then maybe we might get a good signal, but I'm not sure about that yet.

Google probably started paying attention to click-stream only when they've managed to get many people log-in to their gmail accounts in their browsers. At least now they could filter out anonymous stuff and also normalize the effects of 'heavy-clickers'. Privacy suffered though. I hope that anonymous paid click stream could work too, but we'll see.