I mean, he's right. What the hell is a farmer doing in Congress? But you're obviously not allowed to say that.
To answer your question. it's called Democracy. A "Farmer" (considering the rest of his resume, are we really going to choose to identify Sen. Grassley as just a farmer?) is in congress because most of the state loves the guy and votes for him. And no, you are in fact allowed to ask that, most choose not to however because they do not want to sound like a moron who's head is stuck up his ass.
How progressive of you. Yes, the farmers should stay in the fields and the factory workers should stay in the factories. Congress should just be the elite who read about them in textbooks in college so.