Managers don't want waterfall. They want a process they can understand (waterfall is great here), that gives them a specific date they can point to as when everything is done and they can start making money. Waterfall says it gives you that.

Of course we all know that waterfall doesn't actually give you what it claims, but it says when it doesn't you failed to manage one of the early phases correctly which means it tells them how to fix things: work harder there.