Pigs and chickens don't eat air, you know.

70% of US soy becomes animal feed. Some of the rest is used industrially, or becomes biodisel. Relatively little US soy becomes soy sauce, tofu, etc.

Soy subsidies, in practice, mostly function as a chicken and pork subsidy.

You'll notice that we heavily subsidize animal feed crops like corn and soy, and spend much less money subsidizing fruits and veggies, nuts, and other legumes like black beans or lentils.