The state has a different concern: making sure you have enough to eat. One crop failure and people starve to death. We as a society have chosen to solve this problem by subsidizing farming ensuring that there are more crops grown every year than is strictly needed. As such farmers plant more crops than people will eat. This ensures that in bad years there is still enough food.
You can argue that there is a better solution to the problem, but don't argue against subsidizes without a different solution.
