It very much is on some TVs. While there are apps in the corresponding store for Roku, WebOS, Android TV, and Xbox, that still leaves out Playstation and Samsung for instance. Samsung has more than 50% marketshare of premium TVs.

While you can install an app by jumping through hoops, it's not an easy one click install which is what average users need. You can install a Jellyfin server by clicking next a bunch of times. You can get your media there by dragging and dropping it into the media folder. You can install the TV app on most TVs just as easily, but for Samsung you need to do all sorts of extra steps.

A quick Google does give step by step instructions on reddit for instance... but it requires users to download a specific version of Tizen Studio with the CLI (which most people are scared of, they need a GUI to use their devices). They need to connect to their TV remotely via that tool. They need to generate and install security certificates. They need to get specific versions of the Tizen Jellyfin app, that aren't managed by the Jellyfin team, from a random Github. Then rename those files to extract them, inject their certificate, rebuild the package, and send it to the TV remotely... all in the CLI.

That is WAYYYYYY too complicated for the average person. Even with the step by step instructions, people skip steps and skim things without even thinking about it. Most people can barely click next a bunch of times to install things without messing it up somehow. Anyone who's ever worked support can tell you that. My parents and probably half my friends would NEVER be able to follow those instructions without messing it up to connect to my server. And that even assumes they have a PC to run the software in the first place, many people no longer have a PC, they just have their phone and maybe a tablet.

On the other hand, Plex has an app in the Tizen store. Emby, which Jellyfin was forked from, also has a Tizen app in the store. Those people can just go and click install and they're done.

Not in my experience, but I keep my Plex up for my brother. I shifted away personally a year or so ago because I couldn't watch at work anymore (despite self hosting, login still requires a connection to Plex... Which is blocked at work). With Jellyfin, I can just auth against my Authentik server.

But how do you encrypt remote streams? That's the issue with JF, outside the home there's no streaming encryption, so what's to stop you from DMCA notices? For some family, were running a Wireguard VPN through Ubiquiti but nobody else can with ease. At least not that I'm aware of.