Governments (and companies, actually) should not trust servers controlled by other governments for their communications. Typically, all companies using Microsoft Teams, GMail or Slack expose all their communications to those companies and to the US government.
Even when using e2ee services like WhatsApp, metadata are leaked (e.g. which politician is writing to which other politician, and when).
Signal is obviously a good candidate for such sensitive communications, with Sealed Sender and soon Key Transparency. However, metadata may leak at the network level (I believe Signal servers run on AWS, so Amazon could look at the IPs of the messages and deduce who is talking to whom, to some extent).
Now, why wouldn't governments run Signal proxies? That would completely solve the problem of the leaked IPs, right? They could even go fancy an integrate some kind of mixnet feature if needed. Signal already supports proxies, so it would just be a matter of telling their employees to connect through the proxy. Or even build a special version of Signal that only connects through the government proxy.
Would that be a bad idea? Why is nobody doing that?