This feature has been a high priority for some customers for several years.  You are far from the first to ask for it.  I can't give an authoritative answer about why it is so hard to implement.  But I believe it has to do with the number of links/references between project areas (or between the project area and server-level data such as users) and the fact that RTC doesn't deal well when those links break or one end of the link moves to a different server.