This is debatable as well. Spells may follow rules, but there's plenty of examples of "it's magic!" all over the place in D&D. Castles built on clouds, islands that float in the sky, rivers that flow through the air, places where time itself doesn't flow correctly, and so on. Sure, you could say "A Wizard did it" with an unknown spell, but what happens when said spell would have to be higher than 9th level? Some campaign settings have had spells higher than 9th (pre-Karsus Faerun, Athas), but does every one?
And that doesn't even take into account weird monster abilities which have no explanation, like the gas attack of a gorgon that turns one to stone. They just...do it. And the explanation is...uh, magic I guess?