It shows a general lack of imagination. C'mon people, you can't go wrong with alcoholic beverages, just mix some real ingredients up and see how we like it.
They're overpriced. A jug of margarita mix is $5 and makes, well, a jug of margarita. A normal can of frozen limeaid concentrate (basically the same thing) is $1 and has the same effect, and stores easier to boot.
They can't be as healthy. The USDA recommends 3-4 fruit servings a day... why not knock one of those out with real strawberries in your daquiri instead of some unidentified chemical sludge?
They don't taste as good. Cuevo mix is the ultimate one that doesn't taste as good, it's ruining what a margarita is supposed to be and I'm bitter about it.