I think there have been some attempts to do so, but they're just not good enough (and/or end up dead after a while).

So Rocket.Chat (which TIL can federate to Matrix)?

People say discord and my God, to be that is one ugly ass program.

Matrix is fragmented too, but it's generally less fragmented in my experience (if you use a relatively well developed client). Part of this is because most people just use Synapse for their server. With XMPP, server implementations support random combinations of XEPs, and specific servers often are missing random XEPs because they're not enabled by default and so on (thinking about ejabberd for instance here, the default config probably isn't what most people want). I also routinely have random compatibility problems between clients pop up with XMPP. As a basic example, retracting messages is very haphazard.

Anyway, yeah, if they standardize on server and client setup for all govt instances, it'd be fine either way probably. The clients may be somewhat janky, but they can probably fix those issues more easily when they're only focused on one client (although unless it's like FluffyChat and cross-platform, they may need to standardize multiple clients) and server.