The official excuse is that those renderers do not include the editing experience they want to provide. But of course Microsoft doesn’t mind that this way they can ignore any standard they want and dictate what features work and what features aren’t available, all without even needing to publish any specification about how it works. The specification is ‘what you see in Outlook is what you get, good luck!’.

I'm as dumbfounded as you, honestly. Even the IE11 engine would be a step-up to be honest.

In other systems, we are still required to use HTML4 Transitional, and Outlook is only part of the problem. Even webmail, which you expect to handle modern HTML, just fails on unexpected ways (no CSS on head, even if it's embedded on the mail, is one of those egregious problems). It's miles better to be the one working on the website than the marketing email.

The two reasons

- if you copy and paste stuff from Office a lot (like Office users tend to do) it's nice that the formatting will be consistent

- being feature-rich can be a bug and not a feature when dealing with marketing emails