Hi Will.. can we know this reason? I
The current structure offers the most flexibility for managing large numbers of forums. It allows for proper isolation of security and settings.
From our perspective, it would seem that there are two lines of code involved: one creates the main forum, and grabs that ID. The next line uses that ID and creates a sub-forum. A simple boolean in the control panel could wrap an If around this second line..
Sorry, but it is much more complicated than this. As Steven mentioned, you have other options. I would suggest searching the forums for "group forums" and you will find several topics where various options have been discussed.