Mondo Question: Why is there always a robot in 'Alien' films?

This is a new segment that may or may not take off.  Basically, I'm looking for feedback on this question.  Hit me back, please.

So, as the title asks, why is there always a robot in Alien films?  This is even in the rip-offs too.  Let's just take a look...

Alien- a key plot-point involves 'Bilbo' being revealed as a robot.
Aliens- 'Frank Black' is cut open to reveal his robot parts (and lots of milk)
Alien 3- There's no new one, but 'Black' is around again- just because they wanted Lance for his other role, I guess.
Alien Resurrection- 'Girl Interrupted' is a robot- just because.
Unknown Origin- This rip-off features Roddy McDowall's assistant as a robot.
Plaguers- This recent rip-off (with some extra stuff) features 'Charles Manson' as a robot.

So....why?  Why?  Is there a reason?