Sure, but it does spread the costs among users and makes them more manageable. One guy shouldering the cost of a search index is less viable than letting users shoulder the costs. Some charge customers as a solution to this, and that works, but then they need a minimum revenue to continue, or have to monetize with investors which usually means changing direction and goals. The other option, letting people host portions of the index, spreads the cost out, and the product gets about as good (best case scenario) as it's utility to people.