Because fundamentally: certain humans desire power to rule over other humans. Even if the fields (metaphorical or literal) can be done more efficiently by robots, they desire that power to exert over others.

So they will keep some ready for those tasks simply to fuel that irrational desire. They will instead use the more efficient bots to enforce work on the fields rather than work the fields themselves. Or perhaps we go full Matrix and find uses for humans that robots can't properly replicate.