That’s because those companies are ultimately owned by the brand they serve. You’ll notice that you never see McDonald’s serving Pepsi, or Taco Bell serving Coke. And you may also notice that Mountain Dew is usually (but not exclusively) sold alongside Pepsi and Sprite alongside Coke, in these fast food places. And you’ll notice that Barq’s root beer (my favorite) is never sold alongside Pepsi, and Mug root beer is never sold alongside Coke. And other examples like that. Why? Because Coke owns Barq’s and McDonald’s and to a lesser extent Sprite, while Pepsi owns Mug and Taco Bell and to a lesser extent Mountain Sew.