I think you're vastly underestimating how quickly culture deviates and develops. A planetary mono-culture would require every person to grow up in exactly the same circumstances. No stratification from class or gender or sex or age or ethnicity. No varying seasons or biomes or climates. Exposed to all the same media at exactly the same time, and all with the same intelligence and personality and ability to interpret it.

In short, the only time a mono-culture makes even a tiny bit of sense is when it's a hivemind. (Or mind-control but that's pretty much the same thing)