IIRC it's a charge to use their relay servers, which you only need if you don't have a direct connection to your media server. Setting up a direct connection isn't always easy or possible especially for non-technical users, and does come at some small expense to Plex, so it's not that weird to be a paid feature. But if Jellyfin meets your needs that's a good thing, it's probably got a better shot at avoiding enshittification long term.

edit: I was not remembering correctly, even direct (remote) connections are paywalled. You'd need to use some kind of tunnel to make Plex think you're connecting locally.