Because Plex provides a way for me to share my server with my friends. They're mostly not very technical, but they can handle "sign up for this free account and install an app and you can stream from my basement like it's Netflix".

Jellyfin has no such solution that I can tell. Stuff like give them access over Tailscale is not the same user friendly option that Plex has. When there's an actual alternative for easily sharing with friends, I'll consider it. Til then I've had a lifetime Plex Pass for around a decade.