Because the DRI developers are really bad at updating the date renderer string. It really does not have anything to do with when the code was last updated. Slackware 13 comes with Mesa 7.5, which was released on July 17th, 2009. There has only been one official release since then (7.5.1 on September 2nd, already in current).
For what it's worth (which really isn't much), the date has been updated in git master and now shows 20090101.