What stops practitioners of Enchantment and Illusion magics from taking over societies?

Question

What stops practitioners of Enchantment and Illusion magics from taking over humanoid societies in your average D&D 5e world?

See, given enough time and resources, they could impersonate practically anyone (and why would they be in a hurry?), and cover up practically any crime or shady power grab move. (Yes, they could likely charm diviners as well, if not through direct magic, then through simple manipulation, subterfuge, and seduction.)

For extra kudos: If something does stop the practitioners of Enchantment and Illusion, why doesn't that something go the extra mile, and ban such magics and banish and/or exterminate such magic users? (Yeah, nobody expects the Spanish Inquisition, as Monty Python said. :))

I really like this question, and would love to see the answers to it, but I'm voting to close since there are so many completely different possible solutions - I don't see any way to constrain answers or choose one as the best. — Miniman, Apr 14 '16 at 07:15
@Miniman Would it help if I added that the more crunch-based the answer, the better? (Sure, it's a rather open Q, but I guess most of the world-building ones are, and relying on crunch may bring in a touch of objectivity.) — OpaCitiZen, Apr 14 '16 at 07:24
Also, I think the answer might be "the same thing that stops charismatic demagogues from taking over societies without the aid of magic". Which is all sorts of things, and sometimes nothing and they succeed. — Erik, Apr 14 '16 at 08:21
I don't know why you think this'll be different from this duplicate question: http://rpg.stackexchange.com/questions/5781/why-arent-magocracies-and-theocracies-dominant-in-dd-settings - do you really expect "5e specifics" to be overwhelmingly in play? — mxyzplk, Apr 15 '16 at 14:41
@mxyzplk The two questions are similar to a degree, but I don't think they were duplicates of each other. More like, they overlap. This Q here focuses on two "schools" of magic versus all the other schools, forms, etc of magic (and concentrates on dnd5 indeed, unlike the other one which is much broader), and is interested in forms of indirect, hidden control of societies as well, not just more explicit -cracies. (But it's on hold already anyway. :)) — OpaCitiZen, Apr 15 '16 at 15:14
Yes, I can see there's some nominal differences in the question, but the answers will be identical. — mxyzplk, Apr 15 '16 at 17:17

score 15 · Answer 1 · edited Jul 21 '22 at 15:30

There are some downsides to Illusion and Enchantment that can easily make it practitioners unable to "take over societies"

Save throws

For such magic to succeed you need your targets to fail a saving throw. Only one success is needed in order to "get you".

Knowledge of what happened

Especially for Enchantment (for most of the spells) the target knows that a spell was cast upon him - at least when it wears off.

Experience

In the case of Illusion, if it became a common practice to create illusions to take over the power, those who had been once fooled by a spell would become more psychotic (probably) and most probably wouldn't believe what seems too convenient and avoid the most basic illusory tricks.

But the most important is...

Abjuration and Divination

As there are Illusionist and Enchanters, there are wizards willing to use their powers to keep the status quo. You only need True Seeing to avoid illusions. Or Dispel Magic to avoid enchantments. That's the main reason why there are no Enchanters or Illusionist dominating the society.

score 7 · Answer 2 · edited Apr 15 '16 at 12:20

Generally speaking, in D&D, even the gods have limits

How many evil magic-users are there?
- Do they need to sleep?
- Are they immune to the steel-in-the-guts syndrome?
Some races (elves) may be immune to charm effects.
What are the good adventurers doing in the meantime?
Some gods and their clergy may object, and they will have very good wisdom saving throws.

In the "real" world no plan survives contact with the enemy.

In the city of Kelvin of the Grand Duchy of Karameikos spellcasting is forbidden. This is in Mystara, one of the first campaign settings for D&D.
The idea that magic is bad for public order is as old as the game.

The case of the scheming evil lich

Bob is a level 1 wizard who wants to take over the world.
Bob, being an evil genius, knows that his powers are not enough to take over the world, yet.
He starts building his network of evil, acquiring new and more powerful spells and items.

During his rise to power Bob makes enemies. The Paladins of the Order of the Sun are crusading against him. Other evildoers want a slice of the cake.
Bob manages to corrupt the paladins and turns them into his personal army of darkness.
After the tenth assassination attempt Bob decides to ascend to lichhood by some foul ritual, just for convenience.
By any means necessary Bob gets rid of his competitors and becomes king of the world.

But the gods are not happy about what he's done with their creation and start sending emissaries to destroy him, make him repent and free the masses from his grasp.
But somehow, Bob manages to become the one and only god and annihilates all the others, a feat never performed by anyone before. Now he's king of the universe!

Assuming there isn't any pesky Great Old One prancing around...

Bottom line
If you want to set your campaign during Bob's rise to power, you can absolutely do that.
If you want to set your campaign in a crapsack world, you can absolutely do that.
If you want to switch from D&D to Paranoia or Dark Heresy, you can absolutely do that :D