That said, there are some places that do use quick mixers to mix up the classics. Garduños, a huge Albuquerque, NM mexican restaurant (as seen in Breaking Bad, season 5!) uses pre-mixed margarita mix from a keg and dispenses it with a typical bartender's soda gun. They've realized that most people who order "well" margaritas aren't that discerning and appreciate the consistency that a factory-mixed 'rita delivers. Garduños serves them by the gallon and the pre-mix saves them lots of bartender time when they're mixing their most popular cocktail.
This would never fly in a nice bar, though. These days, most big city bars are chasing the money and the money is in "craft cocktails". It's all about the ingredients and the customized drinks that are tailored to the customer's preferences. Bartenders are making a big show out of things like twisting a slice of orange peel to release its oils. The presentation is hugely important here and a robotic mixer just doesn't have the right vibe, in my opinion.