Networking is a big aspect. I have almost 40 friends on plex, about 10 of them actively use my library. I also have access to 8 other plex servers in my circle. And I can put all the "latest added episodes" up on my homescreen with a few clicks.

With jellyfin I'd have to have at least 8 different accounts on 8 different instances.

And while the social aspect isn't great, I found a few interesting people by looking at plex reviews of recently airing shows. Or just finding people through "friends of friends".

There is a lot of things to be gained by having a central account and a connection beyond just very selective accounts on your own server, it really shouldn't be that baffling.