Why do they need Chemicals and Fertilizer every year ?
Short answer is that big farmers don't practice crop rotation any more. Corn is the cash crop, and it requires a lot of nitrogen in the form of ammonium nitrate.
Wheat farmers try to get two crops a year from the same soil, same problem.
Used to be that farmers would grow a crop of soybeans to fix nitrogen into the soil, but that means no corn or wheat for a season, which is where the money is. ,
Because the value of the fertilizer exceeds the cost. The farmers get more money from selling larger harvests than they pay for the fertilizer.