Our evidence is much, much, much better in the case of people. We talk to them, dance with them, have sex with them, have a lifetime of interpreting subtleties of their tone and body language and power politics. We are the same species as they are.

It makes sense to talk about not knowing the mind of a cat or dog in a way that it does not make sense to say the same of a person, even if you grant that "you never really know someone else," and even if you grant the technical possibility that everyone else is just a sophisticated automaton (though it's unclear what that would even mean, or how that would change anything).