There are over 300 Forgotten Realms novels.
Even if you take out the villains who are in more than one of them, survived the events of the stories, or the ones who got resurrected, the vast majority of villains in those novels died and stayed dead. The same goes for the heroes who died during those adventures.
Also resurrection is not as accessible in-universe for characters as it is for PCs.
In the City of the Spider Queen campaign the main villain is a powerful Cleric but the adventure outright states that only a few villains are likely to be resurrected.
Ed Greenwood when talking about the laws regarding resurrection and nobility in the Forgotten Realms on the Candlekeep forums.
It's prohibitively expensive, requires a powerful Cleric, etc.