3 reasons:
1) They're melee and thus cannot be positioned with /follow.
2) Their main attacks are positionals (Ambush, Backstab.. all the tasty stuff) and thus impossible to use in PvE without a tank (which you'd then need to control, too).
3) They're slow. Most other melee classes got pulls with low cooldowns that'll place the mobs where they need it, but while rogues can use ranged weapons that'd cost them their Ambush.

Simply ill-suited for multiboxing. Only way to do it would be by stunlocking which would soon be blocked by trinkets and spells while leaving you wide open to any other attackers (5v1 it'd work well, but what wouldn't?).