VCs have a fund (basically a big finite amount of cash). They can make X investments with that cash. They also have a very finite # of hours they can invest to help their companies. The best ones (like these guys) are literally buried in highly qualified deal flow. That means entrepreneurs coming in with great references, working in great markets, often with traction. They have no shortage of deal flow.

On the other side of the marketplace, there are literally zillions of entrepreneurs who want their money.

All that said, it'd be nice if they had an auto-responder on the site saying, "We try to look over all of these, but can't offer a reply unless something REALLY catches our eye," to set expectations accordingly.