Aliens, why do you look so silly?

[DoPo]

It ties into the whole uncanny valley idea: if something looks human enough, then we'll relate to it easily. If it looks too unsettling or, pardon the pun, alien, then we will find that thing uncomfortable to watch, even if it is supposed to be portrayed sympathetically.

I feel I have to point out that you're not really describing the uncanny valley. The uncanny valley is when things look more human than artificial and people find that more uncomfortable because the minor flaws stand out. The xenomorphs aren't in the uncanny valley - they are totally alien (yeah, pun)

Spoiler: this, however, is there

Spoiler: here is a chart

The dip to the right there - that is the valley.