Hot take: It's symmetry. People like symmetry
People in general? Or many of the people who are the core market for D&D (which overlaps to a greater than random extent, I think, with the core market for other fantasy genre entertainments, sci-fi; and which, I think, includes a greater-than-random representation of engineer-y/tech-y types).
And only semi-related: it surprises me that a system for framing moral and political conflict
in a fantasy game - namely, alignment - gets treated by many D&Ders as
a total system for moral classification of human conduct. Under even the least scrutiny that proposition is absurd: eg Gygax, in describing "good", casually interchanges rights-type, value-type and Benthamite-type descriptors of the good, and in the values includes all the classic ones like dignity, autonomy, truth and beauty; so that, say, a character who accuses someone of being expedient in their willingness to trade off (say) truth for comfort is apt to be contradicted by a Know Alignment spell, which tells you that this is all just happening within the domain of The Good.
Or, to pick examples along a different axis: Gygax links the goal of self-realisation as the path to happiness with CG, yet requires all monks to be lawful; identifies individual freedom with Chaos and regulation with Law, and hence can't even coherently frame questions about the rule of law, constitutionalism, etc - which tend to dominate many contemporary political discussions.
Once it's recognised that alignment is
not, and is obviously not, a total system of description, the idea of alignment-based outer planes becomes pretty ludicrous as a default (it might be fun as a particular take on the issue). And the idea that one would ask "What alignment is Darth Vader" or "What alignment is Batman" or "What alignment is Elrond", as if all genre characters and their conflicts fall under Gygax's idiosyncratic schema, becomes equally absurd.
I feel that 4e was distinctive in having some of the most tightly engineered mechanics of any RPG ever, but at the same time moving away from some of the more simplistic ways in which the D&D story elements had been presented. In those parts of the game which seem to need less engineering and more storytelling (in a rich sense of that term), it didn't hold back.