Maybe you have met the wrong ones.
One of the smartest dogs I've seen was a Shih tzu, although he didn't know that. He belonged to my friend's son. That is the only one I've 'known'.
My Cooper has some Shih tzu in her, with Maltese too. She has been a challenge, to say the least, and by far harder to work with than my Yorkie. She is TOO hyper! That is not because of her breed at all though. All her behavior is from her previous home.