Because tables can be a fast, simple, effective, compatible, and lightweight way to style a page if you don't need fancy CSS behaviour or ivory tower purity of separation between content & style.
Or because you don't know CSS...
Features such as vertically centering elements of unknown height, and mixing absolute with relative positioning are difficult and clunky to implement in CSS in a backwards-compatible manner (IE7 and IE6). I'm talking about: <td width="*" valign="middle">.
This site is a classic example of not over doing anything. It is about conversation and knowledge, and NOTHING else. An extra minute on table-less design would be a waste.
Funny thing that they design with tables but use getElementById for the JS when document.images would have been enough.
I think because pg knows tables.
Minimum viable product = tables.
That's a huge oversight. The technical debt accrued by starting out with a table-based design isn't worth the half hour of coding it saves you.