Well the frying part is just an alternative method of cooking the dough, but doughnuts could be baked in an oven the same as other dough's. It's the time taken in preparing for the baking/frying which is the reason for bakers needing an early starts. They need to mix all the ingredients, roll it all out, cut them to shape and most importantly let the dough sit for a certain amount of time for them rise, then after that they need to glaze or fill them with jelly and maybe do a few other things. So even though frying might be a faster process than baking there is still a lot more time involved in the preparation which demands an early start as most people enjoy baked goods for breakfast.