The answer is not that they are PCs. The answer is that Elminster is busy countering Fzoul. The Simbul has her hands full with Thay. And so on. These NPCs are busy with their own plots and the PCs are the only ones that can handle this plot. If they fail, the world ends for them AND those NPCs. Nobody is going to come in at the last moment to save them and the world if they are about to fail.
I agree. Between the amount of Villains doing stuff and the amount of Villains waiting for someone else to do something so that they can get a surprise attack, it seems like the Realms getting destroyed is just a normal Tuesday.