Think of it this way - why didn't we all just replace Facebook with "host files on our personal sites and give each other access to them" ten years ago? Because we'd have to store passwords, identify each other past spoofed emails, figure out when/how to trust friends of friends, etc. In practice that means everyone adopting the same protocol, which is a chicken-and-egg problem.

That hasn't changed, but what has changed is that reading docs and writing the glue code is effectively free. The main problem with something like Solid is, if it takes even an hour to figure out how to get it going, I am effectively assigning my friends an hour of homework to be able to network with me. That's no longer true. If you like the Fediverse and I like Solid and Dave uses Nostr and Sally is stuck with some proprietary thing that came with her NAS, we can all build a network without any of us switching to, or even learning much about, each other's platforms.