

Probably because it works well, and has working clients on everything at this point. For some, a one-time fee was worth it when it was cheaper.
Sharing is also easier, as your friends just sign up to a plex account and you share your library with them. No need to send them an ip address and port, or fqdn that you have to maintain if your isp changes your ip address. It has its benefits, tbh, and the core sharing features still work for streaming. All the extra crap you can just turn off.
That why I think its still popular.
Same with me, 12 years, about $70, and it still works just as well as ever. I turn off any new features I don’t want, my friends and family can still stream from me for free since I have plex pass already, and it’s easy to share without having to pass around my IP address.