It is likely a combination of sanity, security and a hope that nobody would need it.
I imagine the forum has not adopted the facebook, youtube subscription and google search style "just show them a decent selection of stuff from those they follow" approach and therefore your thousand person follow list generates a serious database query or database load depending upon how it is coded.
If someone really needed to it would probably fall under tough shit. I don't know if mods are subject to such things like the difference between staff and users for using the search repeatedly, and if a user really needed such things (I don't see why but hey) then it might be that a special user group could be made for them.