Except they can and they do... It simply isn't an analogous 1:1 representation of what a human smile means. It's not anthropomorphising to say that a smiling dog is happy, but it is if you suggest that it can only mean happiness. Dogs will smile when they're calm, content, and happy, but they will also smile when they're being submissive or scared, it has to be taken into context with the rest of the body language being displayed and the circumstance.