I asked a good friend of mine who manages a flight kitchen at LAX.

While his flight kitchen does not cater the US3, he said most likely UNITED purchased the tub (my word not his) dressings in bulk and distributed them to the various flight kitchens throughout the system.

He said to have the flight kitchen make the dressing is surprisingly expensive, it is even surprisingly expensive to have the local flight kitchen take a premade dressing, transfer it to a container,load it in a galley cart and board it on the airplane.

The corporate chefs at UNITED probably created the salad with a certain flavor profile in mind and some bean counter said, woah find another way to accomplish the flavor profile and the chef found a tub dressing for much less.

Not an excuse for UNITED being cheap, merely a potential explanation.