Talk politics.

Neo-colonialism persists for the simple reason that it's easier to remove the direct institutions of Empire than it is to remove the economic things that propped it up. Another interrelated aspect is that empires built the structures they created for the benefit of people in the metropole, not the subjects of that Empire. Hence the persistence of what was after it theoretically disintegrated. As to why it existed, the simplest rationales of them all apply: them that built Empires had the brute force to do so, those they conquered did not have the force to stop them.