I laid it out elsewhere in this thread, but in short, costs grow non-linearly with scale: you can run thousands of users on a RPi, but a million users requires whole datacenters. Decentralization not only helps with not requiring "whole datacenters" in the first place, they also enable maximization of resources: if you have a NAS at home, or a RPi hanging around, a router idling somewhere, or an abandoned smartphone in a drawer, you can probably host enough accounts for all the people that you've ever met in your life. And there are hundred of thousands of such underused devices everywhere, which, put together, would be sufficient to host the whole world multiple times around.

The other issue is sustainability: with this centralization comes single point of failure. It's no big deal witnessing the disappearance of one or few providers of a federated network. Accounts and data can be migrated easily. For most users, it's invisible. Now compare this to Signal running into financial issues: you are contemplating million of users losing access to their account and their data, and having to re-bootstrap their whole social graph elsewhere. This is another level of "cost", or price to pay, for centralization.