The problem is... When you have a city ontop of a place of great essence, there start appearing small "Essence" factories, which captures essence in small capsules, and sells it on the black market, this essence screws over the user, but gives a highly prized euphoric rush, just like most drugs, but with magic!.
So to avoid unnecessary addicts, essence related deaths and a general rise in unrest, the AI knows to avoid it!.
ok I am joking 
, good find!
Sincerely
~ Kongdej
