Presumambly the PDF doesn't include the date because it is camera-ready copy for a journal (in this case the filename includes the year of publication).
In general, I'd say people are sloppy about this because publishing is like a super-public form of notarization, which has the effect of placing the name of the article into public indices and databases, most importantly the list of contents of that journal. It's not that people don't care about the date—it's that they have no difficulty finding out what it was.
I observe that the same is true for the paper's authorship information, but nobody seems to have any difficulty placing that prominently near the top of the document itself.
In general, I'd say people are sloppy about this because publishing is like a super-public form of notarization, which has the effect of placing the name of the article into public indices and databases, most importantly the list of contents of that journal. It's not that people don't care about the date—it's that they have no difficulty finding out what it was.