That’s like asking why this tractor trailer truck still uses the same old boring shipping containers from 40 years ago.
My point is that most people in 2025 are expecting a lot more from their communications client than what was available in the "good old days" of Gtalk and FB interoperability, and yet most XMPP advocates just bury their heads in the sand and say "it works for me and does everything I need, so there is no reason to add more features".
The whole argument at the top of the thread is "it doesn't matter how easy/good/performant it is to run a XMPP server, if the people you'll want to talk to do not have a decent client".
That has stopped for me since I started using the element X client (mobile) and Fractal (desktop), which leads me to my "counter argument", which is: Matrix has always sucked in different ways, and it sucks for everyone equally, but at the least it is slowly and constantly sucking less than it used to. XMPP is adequate (not great) for some (Android, Linux) and disgraceful for others (iOS), but unlike Matrix there is no visible progress for those on the sucky side of XMPP.