I would guess primarily that it has not been coded in yet. To really see them be a fair replacement for NPCs which, provided the code is active get paid every month, you need to implement some form of degredation or random chance breakage, or slower working speed. None of which has been coded yet though, so they would currently be an unfair advantage.
While reading your post, Jevon, I got a pop-up in my mind about droids needing a power-charge. So maybe a timer would work, a droid can be active for X hours/days and then a "cooldown" starts counting till the droid can be used again. I'm out loud thinking, this is no official suggestion or whatsoever. :)