Not the anon that made these but I think its because of how 'alien' they are. Most aliens in fiction follow similar conventions but these are truly unique. In each one there is only a small recognisable feature (an eye, a mouth, etc), and the lack of animals they can be compared is really off-putting, giving an uncanny valley effect. It really helps that you can't even work out what some of them are. The one on the right in the 2nd picture has to be my favourite with it's bio-mechanical look and how threatening it appears - my mind instantly thinks of a dog when I see it, but its twisted form is what gives it that uncomfortable feel to it as I realise that it certainly isn't a dog. When I look at them I see them at minifigure scale as well, making them even more imposing and unnerving. Imagine being a minifigure and coming across these fuckers.
Maybe I'm reading too much into it, I don't know.