If I remember correctly, I think it relates to the server-load balancing techniques, and is the same reason the automatic log-out inactivity-timer is set differently for the three log-ins -- DB, forums, wiki.
When considering only those users who log-in (and not those visitors who browse anonymously), the forums are busier than the DB, which is busier than the Wiki (if I remember correctly). This means that the server-load to maintain the connections varies for each server. This "causes" the server admins to set the inactivity-timers differently, to minimize unnecessary load on the servers.
For example, the inactivity-timer that logs people out of the forums is much shorter than the inactivity-timer for the wiki, because it is much more likely that someone will log-in to the forums, and post a comment, and then forget to log-off, compared to someone who logs into the wiki who might be "slowly" editing their pages-of-interest.
In short, a unified-login might be possible, but would increase the server-load by having "unused" connections persist across all three servers, when the user is only using one of the servers.