Having a few really good pieces of work to show off is no guarantee that you'll consistently pump out good work, or that you haven't left clients high and dry before. Since a third party has no real way of gauging that except by your track record, they're taking more of a chance on someone with less of a track record. More risk means they need more reward, which means you get paid less.
Most of us COULD run a Fortune 500 company as well as the average CEO but we won't get the chance, nor compensation, until we have the experience.