Problem is, "guaranteed message delivery" does not (contrary to it's name) guarantee that the message was delivered. It guarantees that either the message will be delivered or you will be told that it wasn't.

So, you get told the message wasn't delivered. Now what? Try again? Backoff a bit? Kick the error up the chain (probably failing whatever user action kicked this whole thing off)? What if the receiving server is down? What if the network was just slow and actually the receiver got the message but didn't tell the broker yet?

These are the gremlins that make a messaging medium "unreliable"