MToF has on page 264 a Necromancer wizard with the ability Summon Undead which says "The necromancer magically summons five skeletons or zombies". Nothing there about targetting corpses. So, own the book, actually read it, still doesn't do what you said.
And sure, maybe it could be a ghast instead... except Ghasts can't turn people into undead.
But wights can! See, if they are slain specifically by the Life Drain attack, not the sword from a double tap, and then... oh, wait. They become undead 24 hours later unless the humanoid is restored to life. Hmmm... revivify could still save them then.
But surely the Wraith can raise someone mid-combat like you said, right? YEs, they could use their action to summon a specter from the corpse by commanding the spirit. But, hmmm... what happens if you kill the specter? Then the target isn't undead (because their soul is freed by the undead being destroyed) and then... they could be revived using revivify.
But why would I possibly nitpick this? It isn't like someone went out of their way to say
Then proceed to give a bunch of house rules that make death more permanent.
Wraith:
Create Specter. The wraith targets a humanoid within 10 feet
of it that has been dead for no longer than 1 minute and died
violently. The target's spirit rises as a specter in the space of
its corpse or in the nearest unoccupied space. The specter is
under the wraith's control. The wraith can have no more than
seven specters under its control at one time.
Night Walker:
Life Eater. A creature reduced to 0 hit points from damage
d ealt by the nightwalker d ies and can't be revived by any means
short of a wish spell .
Son of Kyuss
Burrowing Worm. A worm launches from the spawn of Kyuss
at one humanoid that the spawn can see within 10 feet of it.
The worm latches onto the target's skin unless the target succeeds
on a DC 11 Dexterity saving throw. The worm is a Tiny
undead with AC 6, 1 hit point, a 2 (-4) in every ability score,
and a speed of l foot. While on the target's skin, the worm can
be killed by normal means or scraped off using an action (the
spawn can use this action to launch a scraped-off worm at a
humanoid it can see within 10 feet of the worm). Otherwise,
the worm burrows under the target's skin at the end of the
target's next turn, dealing 1 piercing damage to it. At the end
of each of its turns thereafter, the target takes 7 (2d6) necrotic
damage per worm infesting it (maximum of 10d6).
A worm-infested
target dies if it drops to O hit points, then rises 10 minutes
later as a spawn of Kyuss. If a worm-infested creature is
targeted by an effect that cures disease or removes a curse, all
the worms infesting it wither away.
Yes, creating undead can be long, or short. But it does not change the fact that a character can be animated as an undead. Incinerated, put into acid and what not. And this is assuming the group fled or was killed. What if your healer does not have a revivify spell available in the allotted time?
And Orcus can cast Animate dead as an action.
You conveniently ignored the Nabassu that if it kills a humanoid, that humanoid immediately rises as ghoul. Or the Devourer, Corpse flower. When I am near my books I can be much more precise. The intent is simply to say that if you get to be animated as an undead, no matter how. You're character is toast. That much is obvious per RAW and RAI. You picked the weak examples I gave because it was easy to debunk but you failed to prove that it could not be done.
And check the Vecna our dear friends at WoTC provided us with. Animate dead at will... So yep it can be done and if you go the 3PP, you'll get even more of these and these are sanctioned by WoTC as they are on DMSGuild and are used by a lot of tables.
Finally, the goal is not to say that you can not save a character from death but that there are occasions where said death is irrevocable.
