Manohar's pretty sympathetic about the infohazard thing, but knowledge of Wishes seems too widespread to eliminate? They've played too much of a role in history? There are, as stated, very few ninth-circle casters, and most people who make it that far without blowing themselves up have some sense of caution.
Her descriptions of the standard categories... are vaguely right, for not reflecting the underlying technical realities? Subjective Reality is an Illusion spell because moving things halfway out of reality relative to yourself is technically dual to moving things halfway into reality relative to somebody else. Evocation calls on forces and elements, bringing them into reality, evoking them. Abjuration prevents something or tells it not to happen or blocks it. Necromancy involves the manipulation of life and death. 'Universal' spells are called that because they don't have any of the technical qualities from the other categories, which means you can cast them even if you're a specialized wizard who's pretty bad at some category of effects.
Spells get copied and passed around down through centuries and millennia. Unusually powerful ninth-circle wizards sometimes invent new ones. Failed spells tend to explode and resurrections aren't cheap.
These spells exist and not others because - the person who made them, wanted that spell to be a thing, and it turned out to actually be possible and also they could figure out how to do it?