Just a theory I've advanced elsewhere, but I think the AI never really feels like it's "on the ropes" because of the (very necessary) scripted cash subsidies the AI factions receive, and therefore sees no reason to accept a ceasefire. Also, at the recommended VH campaign setting ceasefires would be tough to get even in vanilla.

That said, as the Romani I was just offered an expensive ceasefire by AS when we shared a border, albeit a remote one in North Africa. They'd just gotten into it with Baktria, Makedon, and Epirus in less than two turns.