Besides, there's the convenience and ease of use aspect. Most of these federated services are not easy to use for the average newcomer or tech-illiterate user demographic, for example. And sometimes they're inconvenient.
Couldn't a federated system that looks like a centralized system work? What if something like Diaspora just installed itself on an AWS EC2 instance for you, and you only had to enter a credit card?
An EC2 instance is probably too much for one user. The model here is Wordpress, which is easy to install and widely used, even in its distributed form. Of course, what's happened is that more and more Wordpress sites run on centralized Wordpress.
Compare:
"wordpress.com" (hosted): "Create your new website for free. WordPress.com is the best place for your personal blog or business site. Create Website (button)"
"wordpress.org" (download): "The latest stable release of WordPress (Version 4.5.2) is available in two formats from the links to your right. If you have no idea what to do with this download, we recommend signing up with one of our web hosting partners that offers a one-click install of WordPress or getting a free account on WordPress.com."
Open source just doesn't get user-friendly onboarding.
Many (or perhaps even most?) web hosts will offer to install Wordpress for their customers, usually with some "Install Wordpress" option either at sign up or in the control panel. And I suppose these installations are configured with automatic upgrades, and are supported to some degree by the web host. Federated and open source doesn't mean that every end-user installs and maintains the software by themselves.
Compare setting up Docker and Wordpress to what Animats just said. Then you'll know why that's hilarious. You've managed to move the problem from one piece of software to another.
Is it as simple as filling out a form on the Internet, getting login credentials, and not thinking about it from there? That is automated and manages itself. What's the Docker + Wordpress version of that?