Plex prices are expensive just to access your own media. Tailscale can do it for free.
Tailscale isn't exactly free. It requires a lot more knowledge, configuration, maintenance, etc, than Plex alone.
Sure, many self-hosters have the ability to figure it out and the proper networking and/or server hardware to implement it. But many Plex users aren't really self-hosters in that sense. Hosting a local media server that deals with all of the networking stuff for you is much easier than maintaining a tailscale or similar setup on top of the media server stuff. I mean for me, if I hadn't gotten a lifetime Plex Pass early on for cheap, I probably would have put more effort into my Jellyfin setup. But Plex mostly just works and I have other bigger priorities. I hate the functionality they've removed that makes things more difficult than it should be, or I wouldn't be switching, but it's not all that bad. So if I didn't have the expertise and hardware already, I could see it being worth the money to stick with it.