I don't think everyone assumes that. For example, much research has gone into whether animals possess a "theory of mind" (ie, the understanding that others have their own stuff going on in their heads, and that stuff can be different from what's going on in one's own head.) There's an entire Wikipedia page for theory of mind in animals: https://en.m.wikipedia.org/wiki/Theory_of_mind_in_animals

Sentience/consciousness is a hard thing to define/study, but that doesn't stop people from trying.

Outside science, it's a fun idea to think about. For example, the obligatory Hitchhiker's Guide to the Galaxy quote: "For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much—the wheel, New York, wars and so on—whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man—for precisely the same reasons."