That's not really true. These clients often reversed engineered other chat protocols and thus supported services that didn't support Jabber / XMPP. This was before e2e encryption was the norm so it was easier to do.

These were all multi-protocol clients supporting more than XMPP because MSN, Yahoo Messenger, and AIM all had their own thing. I believe Facebook was always its own thing but they exposed an XMPP proxy. Only Google Talk really relied on XMPP from the start but it was pretty much a token player in the market until two things happened: 1. They integrated with Gmail making Google Talk readily available to Google’s entire Gmail user base without installing a separate Windows-only client (or finding a compatible messenger, iChat ended up natively supporting XMPP within a couple of years) and 2. Instant Messaging went into rapid decline as users switched to mobile-first messengers (IMing was always a desktop-centric experience and the various pre-existing protocols were not battery efficient and you typically had to have the client open and active to be signed in).