I'm guessing you've never talked to any farmers in the US about pesticides. Pesticides are a HUGE deal. There are limits to the strength and amount of Round-up you use. To be able to purchase large amount of pesticides you have to attend classes every year or two. These classes cover safety of pesticides, strength of pesticides, disposal of pesticides, amount of pesticides a farmer is able to use and any changes in the law or regulation. Frankly, most farmers want to use the minimal amount of pesticides to get the job done. For every amount they spend on pesticides, they have to be able to justify that by the increase of production because a pesticide cuts into the bottom line.
Typically 1 gallon of round up is applied for every 2 acres for most Round-Up Ready crops. A small farm may have 200-400 acres of crops to farm. Round up is not cheap. I typically buy my Round-Up at a local supply store and pay between $80-$90 a gallon. It's been a while since, I've seen the numbers for commercial purchasing, but I'd guess in the $70 range. For a 200 acre farm you'd need 100 gallons, or $700 worth per application. If you thought using 3x as much would be better, the farmer would be spending an extra $1,400. Plus, if you were caught you'd face fines and penalties, and sometimes you cannot even legally sell what you've grown. For most farmers it's not worth it.