Different niches. Jellyfin's a server first, with a built-in (but I think, technically, removable) web UI, and optional clients for a variety of platforms, including set-top boxes like Roku and Android TV and (I think) Apple TV—and Kodi, actually.
Kodi's for running on your set-top box, mostly. Like, yes, you can link Kodi instances together and stuff like that, but it's mainly for running attached to a TV.
I've tried with Kodi off and on, never really being happy with it, since it was still XBMC. I much prefer the Jellyfin model of operation. I spend a lot more time using it and a lot less time having to mess around with it, while using more features than I ever even tried to with Kodi.
I even use both, Jellyfin as a backend and easy web interface, Kodi as a frontend on my htpc thats kept in sync (Jellyfin for Kodi plugin). Best of both worlds. Mainly because streaming JF from Firefox doesn't seem to support surround sound, maybe it's a browser limitation?
XBMC (now called Kodi) still exist and is open source. I’ve been using it for going on 15 years now and it’s never disappointed.