Yes. Is there a good reason for agriculture to be less concentrated than industrial sectors, for example automotive?
I don't think comparing agribusiness to other industries is going to help. Yes, factory farming is more efficient, which makes it cheaper. But the price we pay doesn't necessarily reflect the actual costs to our health and the environment.
Additionally, large farms seemingly are more well suited for polyculture (i.e. small farms are more predisposed to do monoculture farming given their smaller scale).