Why is Necromancy evil but Conjuration is not?

[Cazero]

Can a toddler understand quantum chromodynamics? Does that make it less true (ish)? More arbitrary?

Mortals in D&D are, from a moral capability perspective, toddlers. The moral rules we have are only the surface phenomena. There is deeper order that we simply cannot see.

If it's about perceiving morality directly, that's bullcrap, because morality is evaluated and not perceived. Then it's a mere problem of insight. And what exactly stops mortal for acquiring that insight?

More aptly, if they violate the laws of logic, it is because our premises are wrong/incomplete. Reasoning from inapt axioms produces contradiction where there is none.

No, wrong premises merely bring wrong conclusions without touching the laws. They violate the laws of logic (specificaly, the law of identity that we use to associate a word with an idea) because they're forcing something completely divorced from morality under the same word and pretending it's okay.

In the Great Wheel fantasy model, morality is as ‘physical’ a process as time and gravity... just one of entirely abstract things that are concretely manifest along with law and chaos, mathematical truths, life and death itself, thought... That you see that absurd in practice is fine, but that you categorically reject even the premise sort of defeats the purpose of embracing a fantastical setting...

No, that means I'm capable of processing those things as no longer matching what morality actualy is and refusing to pretend the word has a different meaning.
Fantasy is about exploring ideas, not masquerading them behind different words.