Uh, PerlMonks uses a heavily modified version of the Everything engine
, not slashcode.
As for why he's doing this, maybe it is a learning experience, in which case he doesn't want to simply copy an existing system. Or maybe he just wanted to write his own so that there is little bloat and he knows exactly what was going on under the hood. There are lots of reasons. (And if all he wants to do is get a forum running, yeah, he should use something already out there, but I doubt he'd be thinking about writing his own if that is the case.)