Because mere humans like to have images, colors and text formating in emails.

Because nothing else has anything beyond the most minimal support for right-to-left languages.

Aside: it's nice that the site checks <bdi>, but dir/direction is much more widely used in practice and should be added.

It's convenient for underline, bold or italics in some long winded messages.

I want to write mathematics in my emails all the time, and sometimes I want to communicate with people who don't yet know LaTeX.

HTML emails are a blessing. Receiving them allows me the ability to quickly glance at an email from an unexpected sender and immediately recognize it as a marketing email or cold sales, and send it to the spam folder without needing to read a single word.

Every reason listed in this subthread, but also tracking gifs and other means of spying on the end user.