Because there isn't. It's leftover geometry from creating the beacons that rotates with it. They didn't intend for part of the puzzle to be solved by clipping the debug cam through the ground people...
And when/where did FD confirm this as "leftover geometry"?
Are there similar "leftover geometry" features relating to other objects rotating within game, such as space stations? If there are not then it would seem sensible to view this mechanism as of potential significance rather than dismissing it out of hand.