I think it's partly because adults want to think we can tell boy and girl babies apart visually. We really can't, can we?! But even on MN, I've heard people insisting that they can tell, and girls under three look entirely different from boys. I really don't think they reliably do, except in terms of clothing? Not on here, but somone told me my DN's eyelashes made it obvious she was a girl. 
She is a girl. I'm fairly sure it's not about her eyelashes.