Have you heard of the “outside-in” hypothesis of eucaryote origin proposed by David Baum and Buzz Baum? In this model, the ancestors of mitochondria originally lived just outside another, larger species of cell, with which they already had a mutually beneficial relationship. In which case there was selective pressure to keep the two species of cell close to one another. And the eventual incorporation of the mitochondria into the larger cell was not a miracle of pure chance after all.