It's because it's not humanly possible for a single investor, be it Dave McClure or a YC partner, to process the sheer volume of pitches that come their way. There needs to be some sort of filtering mechanism, and endorsements/recommendations are one way of doing that.
As Dave mentioned in the video embedded in the article, he WILL give his full attention to a founder IF they have already met with 500 Startups portfolio companies (i.e. people he trusts and respects) and proven to them that they are legit.
People like Dave McClure (and pg) have thousands of people vying for his extremely limited time, and so giving his full attention to every pitch that comes his way would be a huge disservice to the other startups that actually deserve it i.e. have actually done their homework, put time in to prove themselves, and committed to forming long-term relationships with people in his community vs. those looking for a quick and easy shortcut to funding and networking.