You don't just average user ratings, you need to have a smart system.
First, each user needs to have a history or they won't be considered in the rating almost at all. Then, each user's history gets analyzed for average rating vs. the average userbase so you can determine whether this user upvotes everything or only rates the sites that are terrible.
So each user gets a score of "threshhold" of what rating site they will thumbs up. Some users will thumbs up a site that's 50% (mostly useless, but not outright spam), while some only upvote sites that are 80% (good content at the very least).
Then regressed for this variable, you can calculate their feelings on each site and see if it correlates well with other users. The more ratings, the more each additional rating is weighted.
This will reduce the number of ratings required for a statistically valid sample for each search query (since you need to tell users if it's relevant for the thing being searched, not just quality of the site). But you also need to fight bots as well.
This system may not be perfect, but it would add value to a search engine