1. http://www.thelawninstitute.org/faqs/?c=183313 2. http://www.epa.gov/agriculture/ag101/landuse.html
Agricultural chemicals are tightly regulated, various environmental groups watch them like a hawk and are quick to push for legislation against new chemicals. The EPA has strict rules on chemical run-off, retention in soil or ability to impact drinking water. On top of this, most farming is relatively low margin and farmers don't want to spray any more than they have to because of cost.
Meanwhile, the average suburban American will gladly dump 10x the appropriate amount of chemicals on their lawn in an attempt to keep up with the Jones' next door. Or think nothing of their fertilizer-infused irrigation spraying half onto the driveway or sidewalk, flowing directly into the storm sewer rather than being absorbed by the soil.
http://earthobservatory.nasa.gov/Features/Lawn/
Which estimates around 31.6 million acres of lawn in the US.