Not only easier to take, possibly hardcoded into diplomacy for the AI to ignore rebels as a negotiating partner, etc. If you can't negotiate with them, lifting a siege won't result in any diplomatic benefit, so you might as well see it through.

Also, Roguebolo, your point noted on "stability" but yes, I do think having AI's expand and compete along with you, to the point where you face factions of similar sizes makes a much better game.