Because in June 2005 the simple response to the Debian bug filed in September 2004 was to comment the global setting out of /etc/login.defs rather than change it to 0027. And after some back and forth there's now the explanation in /etc/login.defs that you can read today (q.v.).
* https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=269583