Or the uber-NPCs are involved in Realms-shattering disasters of their own. Events such as fighting suddenly reactive hoards of orcs and drow in the frozen north (lastest collection of Drizz't novels), wandering around and keeping the Weave from fraying and falling apart by anchoring it to major wards (Elminster in the beginning of the Herald novel), keeping Cormyr from being overrun by Shaddover agents and heading to visit the dragonborn lands (Brimstone Angels novels), readapting to the world after being held captive by an archdevil for a century (Everis Cale), or keeping Tiamat from taking over the place (Minsc and Boo).
The uber-NPCs are kinda busy, which begs the question - why the hells are there constantly such huge events happening? That's the part that stretches my incredibility.
Agreed. Also, the frequency with which they apparently deal with Realms-shattering disasters successfully is one of the classic Mary Sue traits, "Incredible Deeds Done Off-Screen".