That's not a fair comparison, unless the restaurant also brews its own beer.
A better drinks comparison would be choosing from the menu between a 'Jose Cuervo Authentic Margarita' and a margarita mixed fresh by the bartender.
I agree with the OP - in a restaurant I would always order a made-from-scratch dish (the house margarita) than a factory-made burger (the premixed margarita).