Like support for dairy industry came before the dairy industry got enough money to lobby... and sustain itself through lobbying.
Same goes with HFCS, and many others.
It's unfair to argue "evil corporations", when these corporations are made evil specifically by the government intervention in the first place.
I can guarantee you, that should US government pull all financial support for the dairy industry and support plant based products - these corporations will move to plant based alternatives. Because their pure interest is to make money the least cumbersome way.