Here are some of the answers I've been given so far:

1) Alien robot mommies.

2) Alien robot factories.

3) Well, when an alien robot man and an alien robot woman love eachother very much, or have a few drinks...

4) Flame's va....er...ya know what, I don't think this one is worth saying. Children might be reading this.

However, my personal favorite answer is the following:

5) They come from weirdos who often steal aliens and squash them into robots and magically form an alien robot baby. They are always babies, even if the original robots and aliens were old because the ages mix and kill each other in a mega fight for dominance over the alien robot body. So alien robot babies are made. ^^