Aliens are as much organical lifeforms as every other species in the universe probably could be
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With that in mind even if we do not know the Alien physiology it could be so that Aliens do not waste "their energy" on things that are not really a threat for them (attack them) or in any way useful to them (for breeding new Aliens/nourishment), but who knows so
... .
Its not a dealbreaker for me and i am fully okay with it but i agree it could be cool to somewhat occassionally see the Alien punch the synthetic with his tail away to get you
(however those animations and logic need more work behind it so i understand to not see it ingame).
As usual my 3 cents ... .