Yeah, they exist. They're in use today. They're being set up to be a lower impact, more precise way of spraying fields autonomously. All the operator has to do is wait for the drone to spray part of the field and return when batteries and/or the application (spray, seed) is low. Refill, swap batteries, and press "continue".

If you can automate the reloading process you can completely eliminate a huge time suck. Seeding probably isn't viable in some soils because you might actually need a drill.