"The best way to achieve a global, optimal monoculture..." Why would you assume that the best monoculture is better than an ensemble of locally optimal cultures

"The best way to achieve a global, optimal monoculture..." Why would you assume that the best monoculture is better than an ensemble of locally optimal cultures? It seems unlikely for that to be true when the variables that cultures form in response to differ by locality: climate, native flora/fauna, terrain, economy, demographics, etc.