Does a pizza contain an oven? No. Does a pizza contain tomatoes? Yes. Therefore tomatoes are an ingredient and an oven is not.

Does a game contain Unity? Yes. Therefore Unity is an ingredient.

The game ships with Unity which handles the rendering, physics, sound and a whole bunch more. Basically Unity is a pizza base, but it gives you a bunch of toppings too. The developer combines the base with the toppings and voila you’ve got a game. Not saying that last part isn’t hard, but a business model where Unity, or any game engine for that matter, is charged proportionally to the amount of installs isn’t a totally unreasonable business model.