Probably because a- people can tell, and b- you sell more to already drunk people, so getting them drunk sooner is better.
So maybe the answer is water down the shots of your obviously drunk customers.
So maybe the answer is water down the shots of your obviously drunk customers.
TBH, I also had to do my own bit of googling because I barely drink alcohol to begin with, but it does look like "at the start", it's not very distinguishable from ethanol in taste and in effect.
Whatever subtle differences exist between them are probably unnoticeable to people who are already drunk, not to mention drinking cocktails with all sorts of other flavors mixed in.