Because it's the evil of weakness. It's the evil of petty treachery. If they wanted to have a campaign where they go out to conquer the world and bring it under their iron fist, fine by me - I've got whole governments and armies for them to overthrow and rivals to contest with for power.
That's not what they want. They want to suck up and be rewarded. I am uninterested in running a game without challenge or dangerous opponents for them to face. If I create a dangerous opponent for them, their instinct is to roll over on their bellies and beg for treats. And yes, he can then tread on them and crush them but that is in itself the problem. I have waning interest in being put in the position over and over of kill everybody or turn the game into Lickspittle: The Servilling.
And regards your implied solution to this, unfortunately there are two problems that result. The first is that the moment the situation changes they will now try to sell him out to whoever he's sent them against. And if their souls are bound to torture crystals or whatever the equivalent might be in my genre, I no longer have a game of intrigue and player agency, I have "you enter the dungeon..."
The hostage in this equation is the time and effort I have put into making interesting characters and setting. I don't want to throw out what I've created or the style of play I enjoy, for players that don't understand that at some point (and farily early) treachery will get you TPK'd.
EDIT: Tone gets lost on the Internet. It may sound as if I reject your suggestion out of hand. In fact, your suggestion would work fine with others, maybe. But alas I don't think it can with this lot.