Why has the world-threatening, evil BBEG of your 3.X game not been hunted down and slain by a CR 23 solar angel, who protects the world and slays the greatest evils?
What is the in-universe reason?
>Blessed with an array of magical powers and the spellcasting abilities of the most powerful clerics, solars are powerful opponents capable of single-handedly slaying mighty evils. They are the greatest trackers among the celestials, the most masterful of which are said to be able to track the days-old wake of a pit fiend flying through the Astral Plane.
>Some take on the mantle of monster-slayers and hunt powerful fiends and undead such as devourers, night hags, night shades, and pit fiends, even making forays into the evil planes and the Negative Energy Plane to destroy these creatures at their source before they can bring harm to mortals. A few very old solars have succeeded at this task and bear slayer-names of dread creatures that are now extinct by the solar's hand.
>Solars accept roles as guardians, usually of fundamental supernatural concepts, or objects or creatures of great importance. On one world, a group of solars patrols the energy conduits of the sun, alert for any attempts by evil races such as drow to snuff out the light and bring eternal darkness. On another, seven solars stand watch over seven mystical chains keeping evil gods bound within a prison demiplane. On yet another, a solar with a flaming sword stands watch over the original mortal paradise so that no creature may enter.
Why isn't a single specimen of the greatest of the angels doing the world a service and expending 1/1,000,000 of their effort to kill the BBEG?