If you're in a skill position where you're difficult to replace, find a few other such people and all do the same thing.

No one's irreplaceable, but that doesn't mean a company wants a few high-profile people to all leave at the same time, or that a company can't be thrown into disarray for some period of time while they try to replace those people and ward off a "rats fleeing a sinking ship" scenario. Also affects managers personally if they have bonuses tied to retention metrics.