Not to be a dick, but chipmunks at 40 yards with traditional equipment sounds like a bit of an exaggeration. The problem with switching your anchor point is consistency. An experienced shooter can get away with anchoring in an inconsistent place if they are very familiar with their equipment and how it shoots. However, with good form and a specific anchor point you will be able to hit the target much more consistently.
To answer the original question my guess would be that drawing to your mouth allows you to look down the arrow and have the arrow in the center of your vision. I imagine anchoring by your eye could create some issues with looking alongside the arrow rather than down it which would mess up your left and right. Not sure if that makes any sense but again, I'm just guessing. I know a lot of guys prefer to shoot 3 under rather than split finger in order to achieve a "sight picture" thats more in line with the arrow so theres probably some merit to your question.