Because it's not a text organised into sections, paragraphs and whatnot with html, it's just a plain text file rendered into one big paragraph with whitespace set to `pre` ¯\_(ツ)_/¯.
It's arguable whether this is better or worse, because now you'll have the text wrapping where needed _but also_ again where it would have on a wider viewport, even though it already wrapped. So on mobile you'll have a full line, then half a line, then a full line, then half a line, and so on, wrapping alternatingly at the natural edge of the viewport, and again at the original line end terminated by a line break.
It probably changes from person to person, but while I find wrapping like that harder to read than seeing it all normally, for code it is still much better than scrolling back and forth for every line.