Farmers do! And farmers also vote. And that's the reason for why they are subsidised. Farmers have a special interest in receiving subsidies, and politicians gain far more by pandering to them than by saving a little bit of money for everyone else.
I presume you are thinking mostly of the west here. Farmers voting has little or no impact on policy. In most developed countries total employment in farming is less than 5% of the population.
You'd be surprised how much a vocal minority (which is a lot more than 5% in agricultural states) can push senators around. Still, even 5% of the population constitutes a very powerful special interest.
