Our eyes are spherical, our retina is circular, but still our eyes can see more in the horizontal direction than in vertical direction. Why is it so, why is the preferred aspect ratio not square?
I found the explanation given below here (my emphasis). It may not be authoritative, but it makes sense to me.
Another way to see this effect is to look at someone's head in profile and from directly above. In the former case you have a clear view of their eyes, in the latter you don't. (Tested on a small sample - one 3 year old male, playing with an iPad).