I disagree that email should be plain text, but honestly I don't think that's really relevant to the question. I read the question as "why CLI instead of GUI", which I think is totally fair. Using a CLI email client instead of a GUI strikes me like using your feet to open jars - maybe you can do it, but it's so much harder for no benefit.
I think it's not the question. One can continue using GUI, and value CLI for its flexibility. E.g. if I'd like to script some routine task, availability of a CLI tools will make it a breath. In the average case of GUI it's either impossible altogether, or requires some ugly user input simulation. Which is like using your feet to open jars - to borrow your comparison
As another commenter pointed out, CLI/TUI isn't that hard. In many cases it's easier than GUI ones. But I have a different purpose. I can configure different pieces (imap for incoming, smtp for outgoing, notmuch & afew for tagging and search, etc) and use it uniformly from a variety of different programs including git and emacs. Not very simple, I must admit. But it's a personal choice. It works very well for my use cases, including realtime full mail backup and offline use.
> Using a CLI email client instead of a GUI strikes me like using your feet to open jars - maybe you can do it, but it's so much harder for no benefit.
Eh? I used to use mutt and now use notmuch. Much simpler to use than, say, Outlook. Not sure what you're talking about being "harder".
