From what I've read ( mostly in this thread) it's how the yeast work. In the presence of oxygen, they eat and happily multiply. Without oxygen, they eat and produce alcohol, but don't reproduce at the same rate when oxygen is present.
When brewing, you oxygenate the wort to kick start the reproduction. The yeast multiplies, then the oxygen runs out and they start making your alcohol.
Alcohol isn't the goal of a starter. The goal is increasing viable yeast and getting them geared up to produce alcohol once you pitch. That being the goal, an oxygen-rich environment is idea for your starter in order to encourage reproduction.
That's why stir plates are used. They keep the yeast suspended and increase the surface area exposed to oxygen with the vortex...