It's likely that clients don't implement the outbox model because it's expensive to do so. And there are no (or at least very few) open-source libraries, APIs, or other such tools that solve the problem in a way that's reusable between clients.

I'm working on a solution by developing a C++ SDK for Nostr clients. In time, other devs will likely do the same for other languages. In time it will support the outbox model.

Nostr needs infrastructure like this, and we need to find a way to monetize it effectively. There are only so many people who will do the work for free.