Because it isn’t doc is docx.

Publishers are pissy about such things. Even self publishing (which is what I do now), the various outlets still have limits to what they will use. Amazon accepts something like three file formats, including their own, and pdf isn’t on the list.

I could just do pdf for directly giving them away to people, but even then, epub is usually a better pick in terms of readability since that’s the standard for actual books since ereaders tend to display it better than pdfs. Most people reading books via files would be using something that can give a better experience with epub vs pdf.