I think there's a difference between an alien race being "humanoid" ( like greys ) and an alien race that looks nearly identical to humans.
again, for me, the latter is much more interesting.......
Its a psychological effect tho. When you see something otherwise normal and ordinary in an unordinary place or in unusual place you don't expect it to be it can make the normal thing seem frightening or unsettling.
this.
I guess I'm putting myself in the position of the humans in the film.
Let's say, in real life, we finally achieved long distance space travel, and we are able to journey to the far reaches of the galaxy and beyond, and we are searching for signs of alien life ( and searching for clues to our own origins ).
if we journey to some distant alien planet, and our "first contact" with aliens is not the typical humanoid/grey alien we've come to expect, but, instead, is a very human looking giant head statue, and a very human looking ( but tall ) race......the implications of that discovery would make us question our own origins ( and our place in the galaxy ) much more so than if we encountered another humanoid race that didn't really look like us.
It's totally unexpected ( from what our traditional image of humanoid aliens is ). and, as Marvolo said, it makes the familiar much more frightening and unsettling.
Not only would we finally discover/prove that we are "not alone" in the universe, we would also discover, literally, that we as a human race are not alone ( and possibly us Earth humans aren't the 1st humans in the galaxy ).
It would be like if we discovered the planet Krypton and met beings like Jor-El and Kal-El and Kara, etc.......
On the one hand, we'd be thrilled and excited to meet another race that is literally "like us."
However, it would also be scary and unsettling....especially if we found out they were our creators.....and if they are hostile.