Why do Shih Tzu's and Lhasa's need frequent baths?

They don't if you don't keep them in long coat. One of my Tzu has very fine hair that does not do well long so unfortunately we had to shave his body - would never recommend shaving the coat. Our other has a gorgeous coat and is kept long, she needs more regular baths but we do bath them both at the same time as the one in short coat has allergies so that helps him. & they must be fully dried with a hairdryer.