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.
Not sure why you are asking this, but there's evidence that plants do feel pain.
At least some can emit chemicals to signal to others of their species that they are infested with harmful insects so other plants can begin producing toxins to protect themselves from infestation. Some also emit a pulse when damaged, which one might think if as kind of like a scream, though it's not anything humans can perceive with our innate senses.
(At least, not most of us. There may be exceptions. Some humans are abnormally good at sensing magnetic north, etc.)
A stimulus reaction isn't the same thing as "feeling pain", which requires a certain level of consciousness. It's debatable if even lobsters for example have the necessary development.
