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> You can plant crops much closer to each others using an hexagonal pattern.

A traditional seed drill, for crops that can be suitably planted with one, plants rows 7.5 inches apart and spacing within the row is even closer. There is really no room for a human to comfortably exist within that space without tramping the crop. I'm not sure I understand how a hexagonal pattern opens up space for a person in that circumstance without reducing the growth area?

> And s sprayer only spray, it does not harvest anything.

For many crops you're not concerned about, even desire, wiping out the entire plant.

> They actually make UAV sprayer, though...

Certainly. Crop dusting has been a thing for as long as flying has been a viable human achievement. Although with larger and larger machines on the ground able to have a smaller and smaller footprint relative to the working area (and precision technology ensuring that the machine obeys an exact footprint), the practice seems to be going out of favour.