The problem: you need those pesky people to maintain and harvest complex biodiverse systems, and they want livable wages! Best call them savages and uncivilized, drive them away in the name of progess and force everyone to use your machines, fertilizer, pesticides, machinery and fuels.
Yeah, you can pretty much only do polyculture by hand, that's true. Are you suggesting we go back to unmechanized farming?
That would be super great for the planet, as it would cause the deaths by starvation of roughly 90% of the population, with the majority of the remaining humans forced into subsistence farming.
Neither corpses not subsistence farmers would cause much pollution. It might be somewhat hard to get volunteers for it though.
On the other hand, you can absolutely maintain a biodiverse landscape AND use mechanized monoculture fields. You just need to give nature time and space too, and that's something most farmers refuse to do. Which is why they should be forced to. I don't see why a factory gets banned from destroying biodiversity, but it's fine when farmers do it.