It's all about attack surface and HTMLs attack surface is huge. HTML in email is strictly unnecessary. Name another popular messaging tool that allows you to craft custom HTML messages. Text, Whatsapp, social posts, Snapchat, etc.

The only people who want to send HTML emails are marketers, advertisers, trackers, scammers, hackers, and that clueless manager who wants the cornflower blue background. (most of these actors are the same people, except for that last one).