I'm not entirely sure they could. They probably felt that simple cooking of pre-prepared ingredients and then assembly wasn't an interesting problem to solve, especially since staff can also handle the other requirements of food prep (keeping equipment clean and sanitary for example).

The addition of providing a superior experience (custom meat combos? Slicing ingredients on the fly) might not have occurred to them. I think that emphasising the high output gourmet burger is the way forward. Especially since McDonald's has already proven that "Masstige" does have a spot in the food prep industry.