The fermentation doesn't happen because the yeast wants to make ethanol. It happens because it needs energy (which it gets from the sugars and such), ethanol is just one of the waste products. And waste products leave an organism's body for a reason. Mammals breathe air, and expel carbon dioxide, which is very toxic to them. It's not very healthy to swim around in one's own feces or urine either, which is sort of an analogue to ethanol concentrations rising in the yeast's environment.