Earlier quoted context omitted.
When it's a silly marketing email - sure. But you'd be surprised how hard you need to work as a sender to ensure that your content will render correctly if your business is actually to deliver information via email. Remote content is ignored by default by almost all modern email clients (since developers got sneaky and started using it for tracking) so a good email with rich content is usually embedding all that cont…
You don't have to work hard at all if you send plain text.
Did anyone ever read the plaintext version of the email outside our company? Probably not - but it was super useful for testing that the content was correct by dumping the full message contents to console.
Would I have been applauded for only providing customers with a plain text email? Nah, you need a really niche audience to appreciate that - I love that audience, but that audience isn't our customer base unfortunately.
The actual mechanics of email formatting are quite simple (it basically hasn't changed at all in 50+ years) so it can be quite straightforward - it just gets difficult when you try and get fancy.