Revenue from taxes and safety.
It is the distillation process which is regulated and taxed, based on the capacity of the distilling unit. It is the capacity that is taxed, not the actual output by volume. The problem with moonshine (illegally distilled neutral spirits) is quality control. It is really easy to introduce keytones into the mix. It is the contaminants in the alcohol which can be poisonous and what gives you a worse hangover. One of the worst is lead, from soldered pipes with a lead based solder. Worse yet is if a auto radiator is used as the condenser, which not only adds lead but zinc and copper oxides as well. All of which are very bad for your health. Metal poisoning is a hard way to go. That said, who is to know if you make your own still? I've made one out of laboratory glassware that could distill a pint an hour or so. If you use corn (or potatoes) for the mash, you will make neutral spirits, i.e., moonshine. An evaporation still is nothing more than a small heater in a covered bucket inside a bigger covered bucket so vapor condenses on the top and then runs down the sides to collect the distillate in the bottom of the bigger bucket. No boiling happens, just warm vapor which condenses much like condensation on the mirror in the bathroom. If you start with sterilized equipment, and prevent any contaminants from the corn or whatever, you will make very pure moonshine, but in small quantities. Moonshiners who make a hundred gallons or more at a time, have a quality control problem that a commercial brewery and distillery does not have. So, the fed rationalizes the law on taxes and safety when it comes to distilling, but NOT on simple wine and beer which does not have the same risks from contamination.