Because many humans don't take the time to understand animals, they see them as dumb creatures. Just because they don't build giant buildings, or enslave people, or treat other animals horrible!
I sit in my aviary, and I watch my finches. To many people, finches are just birds with tiny brains, who just eat, poop, and tweet. Because I spend most of the day in my aviary (on the weekends) I can see the intelligence in their eyes! There was one finch, who was sitting near my legs. He looked up at me, and after a while of watching, he went over and started plucking the leg hairs out of my leg!
Some humans are a bit mental, don't you think?