Tractors are slow and cumbersome. Field conditions or wind usually keep them out 50% of the time when you're ready to go or they'll mess up the row crops. So if you farm 1500ac you won't be able to get all your fields done during your windows for treatment (varies from a few days to a week where I am from). A spray rig costs 300k. For that price + the cost of an ag kit, you could buy an OH-58 and be able to work 2000+ acres per day. So most farmers just hire somebody to do it. For as much as they pull in for crop, its just a drop in the bucket.