Sounds like you found the answer and made an account to complain?
To answer your question about why we don't have a better un-installation method on linux. People that contribute to Joplin don't uninstall, and people who uninstall don't contribute. That said, it wouldn't be too hard to add an uninstall option to the script, I'll add it to my list (someone else feel free to do this first).
As the other posts on this forum say just delete
~/.joplin // Contains the executable
~/.config/joplin-desktop // Contains configuration and notes
~/.local/share/applications/appimagekit-joplin.desktop // is the desktop file
btw Joplin has an experimental WYSIWYG editor (not markdown which you might be interested in. Feel free to check back in a couple months.