When I send someone a Word document, I can't guarantee that it will look the same

Exactly! Isn't that wonderful? People can view it on a desktop, a phone, a text-to-speech engine if they want to have it in audio form... it all just works because the tags are preserved that show what's supposed to be a heading, where new paragraphs start, whether text to the right is a side box or a continuation of the line, etc.

certainly more consistently presented to the end user

   We all love horizontal scrolling to read your sentences, yes. Let's make everything preformatted text with a line length chosen by the sender. (Yes I know HN now inserts line breaks in code blocks now. Isn't it nice to have reflowed text?)