I’m very disappointed to find that there is a lot of shows I can’t watch on plex, even with a plex pass? Why is this?
A Plex Pass won’t give you special access to additional content on Plex‘ video-on-demand and live-tv streaming services. While Plex has features to show where you can watch certain content, that doesn’t make that media available (e.g. if a show is only available on Disney+, you can still only watch it there).
https://support.plex.tv/articles/200288566-what-is-plex-pass/
Plex is not an illegal streaming service. It has some free (ad-supported) TV and Movie offerings, but they are usually obscure and/or older. Plex Pass provides near-zero benefit to someone wishing to use Plex’s ad-supported media offerings.
The primary purpose of Plex (and it’s associated Plex Pass) is the creation of your own server to provide streaming capabilities of your own content.