PageRank was magic before people learned to game it
Even then, before Google started going after them by filtering results, the SEO spam sites were pretty easily recognisable and ignorable as you scrolled through the results. Now, they still show up in droves (try searching for service manual PDFs and you'll instantly see what I mean) but you hit the end of the viewable results far too soon to find the useful stuff buried in later pages.
In other words, Google's ranking now seems to be "good SEO'd sites > spammy SEO'd sites > everything else", and cuts off results before getting to that third category, when that third category should ideally switch places with the second and maybe even the first.
Even then, before Google started going after them by filtering results, the SEO spam sites were pretty easily recognisable and ignorable as you scrolled through the results. Now, they still show up in droves (try searching for service manual PDFs and you'll instantly see what I mean) but you hit the end of the viewable results far too soon to find the useful stuff buried in later pages.
In other words, Google's ranking now seems to be "good SEO'd sites > spammy SEO'd sites > everything else", and cuts off results before getting to that third category, when that third category should ideally switch places with the second and maybe even the first.