Because the aggregated weighted result ranking provides a more useful page rank than any individual search engine, and if any search engine tries to (accidentally or otherwise) stuff specific results into the top ranks, it doesn't matter. It'll be deranked because no other engine displays those results highly. In a similar manner, it deranks targeted SEO attempts unless multiple platforms are targeted.
Don't get me wrong, it still has its problems. For example, if the individual search engines all get a bit too samey, then it will as well.