Not only is it infeasible, but there's so much garbage out there that we have no simple way to filter it out from scratch to the point that anyone could actuall

Not only is it infeasible, but there's so much garbage out there that we have no simple way to filter it out from scratch to the point that anyone could actually use it. Google and Bing have feedback loops with their users that prevent crap from rising to the top of search results. A plain index (or even using something like pagerank) wouldn't have this huge benefit, and you'd never be able to get your search engine off the ground.