I agree. This is a peeve for me. I ripped toons for my kids and I can’t get the movie or to show view to display them appropriately. A directory view would have been perfect
If you have say "movie.mp4" then make two copies of a jpeg named "movie.jpg" and "movie-fanart.jpg" alongside the movie file. Plex will then show this.
You could generate the jpeg using ImageMagick or similar from a base image (per show say) and with text overlay.
I have some scripts that does this and it's better than nothing.