How do you know that? Descartes believed that animals couldn't feel pain. We're all shocked by that now, but he could have (in fact probably did) use an argument similar to "that's anthropomorphising".

There's a long history of humans making claims about what animals don't experience, in order to justify how we treat them, only to later discover evidence that overturns the claims. It seems to me that this has to be the default explanation for zoos as well.