Besides, if you pitched a vial or smack pack in a full 5 gallon batch of beer you are underpitching, but pitching the same into a starter you are way overpitching--so it takes less time for it to reproduce to the level it can reach in the starter. The goal of a starter is not fermentation, but reproduction. There is no benefit to letting the starter go beyond the reproduction stage--you want it to do the work of fermentation in your beer. Finally, the yeast take in oxygen during reproduction, but during fermentation they use up the oxygen that they have stored. By the end of fermentation they are oxygen depleted. If you waited until fermentation of the starter was finished you would be pitching the yeast in a low oxygen state.