very, very early on in Signal, back when it was still called TextSecure, they did actually federate with other servers, in particular with servers run by CyanogenMod (predecessor of LineageOS, briefly a commercial startup, now defunct).

https://signal.org/blog/cyanogen-integration/

It seems like this was a very bad experience for Open Whisper Systems which is why they don't like decentralization.

Maybe the arguments for and against have changed since then.

But it's not like they just refuse to try it for no reason.