You're moving the goal post from "self-sovereignty" to supports federation with an infinite number of servers. Nothing is stopping you from compiling your own Signal server and modifying a Signal client to use your server.
Given that Signal is free as a service, supporting federation only increases their expenses.
Because centralisation provides ecosystem agility, which they absolutely value as an upside. Find a way of doing post-quantum secure key exchange? Just roll it out to the server and all the clients essentially overnight.
I'm well aware of their justifications. I'm also aware that centralization brings systemic risks, which they don't talk about.
The internet would be a lot more efficient and able to evolve if we just had it controlled by one single entity like Google or Microsoft. Do you think is a good idea to do that?
The economy would be a lot more efficient and allocation of resources could be a lot more fair if we could put it all in the hands of one single corporation or government. Do you think it's a good idea to do that?
Agricultural output would improve significantly if all crops used the exact same genetic strain and if all soil was artificially managed. Do you think it's a good idea to do that?
In case you are wondering, "ability to quickly roll out post-quantum key exchange" is waaaaay down the list of my worries compared to "facing a catastrophic Black Swan affecting all of the world's communications".
Given that Signal is free as a service, supporting federation only increases their expenses.