You learn eventually how to avoid the crap.
There's also forums and stuff where people congregate and provide suggestions regarding the good stuff. Plus most aggregate sites tend to have favorites and stuff. Once you find one author or reader (there's often fic-less profiles that just contain a honking huge favorite list) that coincides with your tastes, you can often ride the wave of favorites for dozens of decent-to-good fics. Basically other people filter out the crap for you, to a large extent
But yeah, while I was cooking I figured out a much easier way to explain the phenomena. Goes like this:
You know what's great? "Twinkle Twinkle Little Star" is great. You know what would be
awesome? "Twinkle Twinkle Little Star"
done on flamenco guitar*. And, you know what? I can do that. I can do that, and then I can share it, and I increase the awesome of the world.
That, right there, is one of the major driving forces behind fanfiction. I mean. You can create something original and make an awesome thing, too, definitely. But in the end, it's still +1 awesome and that is a wonderful and oft-times fulfilling thing. Is the person who makes a good thing great really all that less of a thing than the person who simple makes a great thing? Maybe a bit less, sure. But they both make great things.
*I know this because I've heard "Twinkle Twinkle Little Star" done on flamenco guitar, and it
is awesome. E: Tangentially, Mozart on snare drum is pretty ballin'.