Because it’s already a proposed feature on their feature voting website. In fact It’s been one of the top voted features for the last 7 years straight. It’s at this point the most often talked about drawback of Jellyfin and biggest stated reason why people won’t switch away from Plex. It’s been so long that the SSO plugin has been archived because the maintainer only made it on a temporary basis and he was tired of maintaining it, likely because it’s only taken the pressure off of the Jellyfin team to implement native SSO because folks like you like to point to it as a solution to the problem.