@monsieuricon I don't know why email providers use custom engines, but I'm not convinced by your explanation.
Web browsers are extremely secure, and most of their exploits are in script execution, which is disabled in email clients. No matter what engine you use, there will be exploits, which includes the MS Office renderer.
I think it is more likely that it is hard to support emails written for MS Office in a webview, and that's why they use MS Office. To keep backwards compatibility.
You mention that the "Email not displaying properly? View it on our website" link is proof that HTML was a bad idea. I think it shows the exact opposite.
Email is not real HTML, it is a proprietary lookalike made up by MS. And it is so hard to use, that senders will go out of their way to serve you a separate copy in real HTML because that works much better. Clearly MS Office was a bad idea, and everyone agrees that HTML is a good idea.