No one assumes that. But in Iowa, the farmers wield influence far out of proportion to their numbers, primarily because so much of the economy in the state is dependent upon agriculture. In other words, it is not just the farmers - it is also the farming equipment dealers, the people who repair that equipment and sell other supplies to farmers, the people who work on the farms; then the people who sell groceries, homes, cars and other goods to the people who rely on the farms for their income, etc.
That does not justify a candidate pandering, but does help explain why the farmers have such influence.
Having lived in Iowa from 1974 (age 14) to 2002 (age 42)(except for my service in the armed forces) and having worked on two presidential campaigns, I’m aware of the conditions on the ground there.
But since there is a slice of Iowans who are against pandering to the farm economy, and since only a few politicians are bold enough to tap into this - this is a wide open opportunity.