I suppose because until you actually show that something is the case, experimentally, it's just guess work either way. The world is full of different kinds of people. You, for instance, obviously see dogs as intelligent mammals. But there's also the person who buys a puppy for their 8 year old, and sees it as a fluffy clockwork toy that eats and shits. Not only do they not bother to wonder if it's sentient or intelligent, they actively suppress those thoughts because they are inconvenient thoughts. Those thoughts mean you have to care how the thing 'feels'.