This falls under world building and rules of magic.
For example, When I worked out my magic system, I worked out how magic works for the entire cosmos, then how it operates on an individual level per universe. I created an entire type of Physics for Magic involving monads as the funidmental partical and the various types. Up, Down, Weird, Demonic, Angelic, Top.
"Free" mana is two up monads and one down monad. Mana is manufactured from "souls". Souls are made up of the other four types, but monads can break down into other types. Human souls are made of top monads and human souls act like Plankton. They provide the "oxygen" that more "higher" memetic creatures need to exist. Why Fae, Demons, and Angels are so facinated with humans.
For the most part, humans manufacture "Positive" and "Negative" mana depending on their emotions. However, said mana "builds up" and then "runs off" This is what produces Ley Lines and Dragon Lines. "Moving" mana does not accumulate anywhere that it can build up to create "Natural" outbreaks.
So, while humans consume "Positive" mana, said mana becomes "depleted" and becomes "negative" mana. This mana becomes "run-off" and flows "downhill".
You will note I use quotes alot, because this is all Metaphor. Considering that their are six dimensions, what is "Downhill" is not what you or I think of. Massive number of humans results in a "Pressure Gradient" in the higher memetic dimensions, so rarely does "negative" mana build up there.
Instead the negative mana "runs off" and will "pool" somewhere. Ths location then accumulates negative mana that will eventually crystalize into more condenced form or monads, usually demonic. Get enough demonic energy, you will get demonic crystals that will start to form bodies based on local prevaling negative memetics.
In otherwords, you get a dungeon.
However, it might be a forest, a hole in the ground, or the top of a mountain. It all depends on what is the Memetic topography of an area and where mana can accumulate. Sort of like a particles of dust in the air can cause clouds to form, so too can crystalizing negative mana result in demonic lifeforms.
However, they rarely form inside cities. They usually pool away from "high pressure" areas, such as cities. If left unchecked, yes, you will get large number of demonic beasts who will attack, but since they start small, it can take a while before the demonic horde gets large enough to attack a city. So, the lack of attacks are usually because the local population noticed this happening, and thus sends in adventures to clear out a "dungeon" before it gets out of hand.