I'm guessing Aliens need organic hosts as they are organic creatures themselves. Synthetics are man-made and no matter how similar in organic structure they may be an Alien might be able to tell the difference. I can just imagine this wierd example: a facehugger trying to impregnate a synthetic would probably be similar to a facehugger trying to impregnate a laptop computer... I know, bad visuals. Lol.
~ No one is taller than the last man standing ~
~ Males conquer, females rule ~