There are plugins for SSO.
There are 3rd party plugins for OIDC and I think LDAP is even first party.
The issue comes when intercepting the signin-progress with 1st party clients. Jellyfin (to my knowledge) doesnt support redirects/callbacks like a homeassistant companion app does.
And how many media servers are there? The 2 other major offerings (Plex and Emby) don't support OIDC either.
Plex does it's own sauce and Emby doesnt support it. Authentik has a guide to implement it via LDAP.
And Jellyfin has a tech-debt history being forked from emby. Stark contrast to newly developed projects which were started when SSO and OIDC wasbstarting to become popular.