If you've ever been on the other end of the pipe for hiring you'll understand the need for filtering. The alternative is to never list jobs and hire through networks, which has other problems.
I've never been on the other side of things, but it's always fascinated me that so many people who have say things like this. It's hard for me to fathom that such a large proportion of candidates are terrible.
It's the natural outcome of there being a very small group of bad candidates who apply for a lot of jobs, and a very large group of good candidates who don't apply for jobs very often. What if there's only 50 bad candidates in a particular region, but they all apply for every posted job opening? That's a lot of chaff to get through on every single job posting.
Things aren't that simple for a lot of reasons, but the basic idea holds - you don't need a huge number of people applying for jobs they're clearly unsuited for in order for those people to add up to a huge portion of applicants for any particular job opening.