But that's because it does way more. Matrix is the foundation to build a self-hosted Slack-like platform (public/private rooms, audio and video support, threads, e2e encryption, etc). XMPP is basically self-hosted AOL Instant Messenger.
Whatever you can think of, XMPP supports it. Whether enough clients support it for it to be reliable is another question entirely!
(The XEPs I mentioned are considered "core modern" so most clients support them, you can browse through https://docs.modernxmpp.org/client/protocol/ to see what's considered the bare minimum for a full client)
> public/private rooms
Fully supported by XMPP: https://xmpp.org/extensions/xep-0045.html
> audio and video support
Supported by some clients (usually over Jingle) but nothing standard AFAIK: https://xmpp.org/about/technology-overview/#jingle
> threads
Rooms can have multiple threads but I don't think they're used the way they are in discord/slack/etc.
> e2e encryption
XMPP has multiple ways. Currently the most widely used by clients is OMEMO https://xmpp.org/extensions/xep-0384.html
> etc
Whatever you can think of, XMPP supports it. Whether enough clients support it for it to be reliable is another question entirely!
(The XEPs I mentioned are considered "core modern" so most clients support them, you can browse through https://docs.modernxmpp.org/client/protocol/ to see what's considered the bare minimum for a full client)