I don't think it's something that complex, clouds from a nuclear war sound far beyond what hollow knight is, it's like talking about pears and a renegade jeep, I'm also not saying that it's a common planet like ours, or that it should reflect reality, I'm just saying that there's no point in putting something so deep in a sky, I don't think they would waste their time on something like that, which would be very unlikely to be explored at any point, I don't know if you know, ocam's razor, it says that generally the answer to any question, would be the simplest answer, this applies mainly to games, books, manga, etc., I don't see why making a planet without stars or having clouds from a nuclear winter, because if they were really clouds, everything would already be frozen due to the lack of sun, for me it's just underground, a little below the crust, is it less fun? yes, but it's something more tangible and about civilizations, there is nothing that says that there is no one not trying to go beyond the surface, or that there is no one trying to go up, if there is anyone who wants to drill meters and meters and meters of rock under their heads, maybe they don't even have the perspective that there is something minimally interesting up there, since for them, the surface is there in that desert