That's what the specification says they should. Just like HTML emails should not have an empty "text/plain" as an alternative to their main "text/html" content, but some mailers do that anyway.

And I do mean empty, not absent, so MUAs don't even try to convert the HTML to plaintext.

Or sometimes, the plaintext version is buggy, eg. because the mailer forgot to remplace {{template variables}}. Or because it sent a boilerplate text. Or the content of a mailing that was sent years ago. (All of these are true stories, I got emails like these.)

The conclusion being that if some format (AMP, HTML, ...) becomes prevalent, developers will stop caring about other formats.