Because the talking animals I know tend to prefer “it”. The ones who were unfortunately born in human bodies, I mean.

And because singular “they” implies personhood, and I don’t want to project personhood onto animals. Instead, I want us to decouple our ethics from personhood and treat non-persons with some level of equality with persons. You shouldn’t need to be a person in order to have rights. Calling animals people just reifies the supremacy of persons, and causes more issues down the line when it comes to the questions of non-persons who aren’t animals, and their rights. Like aliens and AIs.