Maybe it's the people responsible for retention just don't have the time to focus on retention most of the time? They're busy putting out fires, going to meetings, worrying about numbers and other things that managers do. They just can't spend enough time on things that keep current employees around longer. Plus, at most places, retention isn't a number that is measured, or if it is, it doesn't matter much.

Since retention doesn't count, it gets pushed aside most of the time, so the people responsible end up spending more time on hiring/recruitment, which gives them less time to think about retention.