The details are there in this post, but I can offer a few guesses. Users may be using multiple devices. And the service has to deliver to all the linked devices before ejecting the message from its storage. The time limit for storing and waiting for linked devices to come online is about a month. With tens of millions of users, this could add up.
shrugs
[1]: It was a copy of this cat video: https://www.youtube.com/shorts/8Ud1Cr76j8s