[quantumofalpha]

wat? Google used clicks ("navboost") since approximately forever, it's one of strongest signals for all major web search engines. I guess the better question is why they don't optimize ranking directly on clicks, why still bother with all that expensive human raters business... well optimizing for clicks helps but only until a certain point beyond which it starts hurting relevancy by overpromoting old popular results and clickbait. But as an ingredient in overall ranking in one way or another they and everyone else definitely use click data.