>>52736977I think you're severely overestimating exactly how large of a volcano Mt. Chimney is. It is still spewing ash, but is primarily doing so north, towards Route 113. And of course, the desert is surrounded by cliffs and mountains, preventing proper rainfall, which is what a desert is. All of this is plenty logical.
If you mean why Gamefreak added those things, it was likely to play into the themes and new features of the region. They can show weather in the overworld, the box legendaries are all about weather, and having spots like a volcano and desert add contrast with the dichotomy of land vs. sea that the game tries to establish.
Again, nothing you've said is establishing what part of this is illogical. It seems more like you're just seething over the fact that volcanoes can exist without destroying everything and deserts form where rain doesn't fall.
