Oops, I meant to type another paragraph afterwards but missed it out.
A 20th level adventurer has part of what they need - the raw power - but to get the power base of allies, they will have to spend time building their network. Usually, adventurers can't or won't spend this time.
The Evil Bad Girl has killed the empress and taken her place, but she now has to spend three quarters of every day alternatively threatening and cajoling civil servants. Where does she get the time to do Evil Ritual Stuff? Sure, she could let her Trusted Second In Command take care of the day to day stuff, but you just know that after a few months of running the empire, the 2ic will get delusions of grandeur. What do do, what to do?
It's true that players are on the whole an unruly and unreliable mob, who show little sign of being able to stick to the commitments that authority requires. On the other hand, that is partly DM driven, because a political campaign demands the group become interested in another kind of narrative, away from the dungeon.
I've played in and occasionally DM'd political campaigns. Once it gets rolling, player character behaviour changes and they become involved with the acquisition and exercise of authority. It's probably not for everyone, although if a group hasn't played that way it can be a welcome alternative.
So there is an interesting contradiction with what you say. The common adventurer is an unreliable murder-hobo. Yet somewhere there is a pivot point that allows someone like EBG to become engaged with a new layer of the campaign world. That can be something to do with reaching a level where the tactical game is not as well supported, but the strategic game becomes very intriguing. She has the wealth to form a guild or build a fortress. She isn't alone, she can share power with other players, or neighbour with them (and get into conflict with them, in my experience).
Most CCEQs are not players of course. At high-level I've consistently found that the most interesting sort of opponents for PCs are CCEQs rather than monsters. And these opponents become vitally interesting when a DM works them along the strategic and political, as well as tactical dimensions.