The FDS story is simply the means by which the GB justifies its power. The reality is that the men on that body are dedicated loyalists who have worked their way up the organizational hierarchy. There is simply no way the GB would share power with anyone who simply claims to be part of the same special group. That would pose too great a threat to their power.
There's a much more complex answer, one that traces the JW development of the FDS doctrine. I really don't know enough to write in great detail about it, but the FDS doctrine was originally used to justify Russell's power over the Watchtower organization. Originally, Russell, himself, was the faithful and discreet slave. The doctrine has morphed over the years into where it stands today. Of course, the doctrine falls apart under critical analysis, but the purpose isn't to have an airtight religious doctrine. The purpose of the FDS doctrine today is solely to justify the governing body's authority.
It is true that it makes no sense to give the governing body sole authority over the entire organization since the FDS, which under the current understanding is supposed to be comprised of all of the anointed remnant, are supposed to be the ones taking the lead, but the governing body gets around that by claiming to be spokesmen or an elder body for the FDS. JWs are indoctrinated to obey above all, so this trick has been very successful in the JW context.