I did not say 'only reason'. Of course there could be other benefits. And I am guessing we are not talking about the same restaurant experiences either.
veggies are real cheap compared to meats. 1 pound of meat or seafood is 4-30 dollars wholesale versus 1-5 dollars a pound for veggies, so after markup, what are most dishes going to contain? Fillers: carbs and veggies.