The voting mechanism certainly does help, but only to a point. Imagine a "new" page where only 1 in 10 posts are worthy of upvoting (about the same as now.) Now, after some growth, imagine that changing so only 1 in 100 are valuable. Then one in a thousand. If you don't weed out bad submitters, it eventually becomes so hard to find the things that are interesting that you either start "grading on a curve" and just upvoting the best of what you can see—thus creating schlock like Reddit and Digg have—or you give up and move on to another site.

To put it in telecommunications terms: you need to keep the signal clear throughout transmission. If the clarity drops, amplifying it again (through a process like voting) will only pick up the bits that survived, and will also amplify some of the noise in the process. If we can eliminate the sources of noise, however, then we don't have to amplify.