Maybe the vent path was originally supposed to be piped THROUGH the furnace area, to the outside, and some of the piping got taken out somewhere along the way, or just wasn't ever installed. In the house where I grew up, the shower fan pushed moist air through a channel that led to a coiled accordion pipe lying in an obscure corner of the attic, with its end open to the interior of the house...  obviously it was supposed to be hooked up to some kind of outdoor vent hole someplace, but instead we always just let that section of the attic be disgustingly dusty, moldy, and (sometimes) moist.  (I only found out about this as an adult; the question of why nobody had fixed it prior to that is outside the scope of this forum, since obesity was not involved.)