Another reason is that powers in RPGs do not line up in nice plot devices Nice and neat from the writer's point of view, of course. The fact that a character would logically have a power that would blast away his latest obstacle is unnice and unneat when the writer wants the character to be stopped. But sometimes you need the character to have a block-buster power and lack another. In Order of the Stick, Burlew has explicitly not given stats for the characters to avoid painting himself into a corner, and even so has had to deliberately make them not optimized. (The tactics of limiting spells accessibility only works for spells themselves.)
Especially when you want to give them an equal but less obviously applicable power and make them sweat for it.
And once you axe the omnium gatherum of monsters, there's no reason why inherent powers should even exist. No undead -- no turning undead.
Less of a problem for non-class-based systems, to be sure.