Page media has been supported for decades. Line numbers aren’t strictly related to page media. It’s hard to complain about the lack of in-depth support for a media that isn’t meant to display “the web”.
I do agree that it could be better. They have been adding print-related features though, at least I remember some in CSS3. CSS Colors Level 4 includes cmyk functions.
I don't think "it isn't meant" is accurate anymore. The browser has become meant for all things including document creation and styling. The @page counter feature although documented isn't supported by any browser from what I can tell at this point.
I do agree that it could be better. They have been adding print-related features though, at least I remember some in CSS3. CSS Colors Level 4 includes cmyk functions.