No, they are not. If you think that you haven't been paying attention.

Ok, show me difference in culture between northern and southern Kaladesh/Kamigawa/Innistrad/PlaneOfHats? What is the difference in culture (how do they dress/eat/behave differently) within a species?

For example does northern Innistrad have 3 month day/night cycles, during which vampire attacks subside/increase? Do Northern Innistrad vampires have tolerance for sunlight? Who knows? It's gothic horror, fuck snow.

Let's look at Guild Wars and look how they do their one world. In north you have Norn/Human settlements that look like nomadic settlements shored inside giant structures of old. In the south you have Egyptian/Indian settlement with Giant Sand Worms and desert of pure white phosphorous.

That said, it's fine to have a planet of Hats, here and there. Some planes are not big enough to host diverse cultures (e.g. Serra's Realm), some of them might be just be a treasure plane for some oldwalker (Ixalan?), but most of them seem to be realms that could and should have diverse cultures, but don't.