Because the yields we get may be better when we use their product.

Warning: Generalized round-about numbers ahead

If I farm 2500 acres (small-scale farm), can yield 100 bushels to the acre, and corn is at $6.33 per bushel; that generates $1,582,500 of revenue (2500x100x6.33).

Now, let's assume I can yield 10 more bushels per acre by using x, y, or z Monsanto product. Now I'm generating $1,740,750 of revenue. That's a difference of $158,250 simply by getting 10 more bushels to the acre. Let's also assume that the total cost of x, y, or z Monsanto product for the operating year is less than $158,250.

Excluding all other factors (moral, social, etc.), if it ends up being more profitable to have to buy Monsanto's stuff every year because I can yield x amount more per acre, then that's what I do.