Because the original words are associated with female. E.g a nurse is generally thought of as a female, so "male-nurse" makes the detail clearer.(Original post by MylittlePlusle)
Man-purse, man-hug, man-whore, man-slut? I don't get it, I don't think it's needed, a purse is a purse, a hug is a hug a whore/slut, is a whore/slut. I don't think gender/sex matters at all.
The same goes with females... e.g female doctor - as a lot of people think of a doctor as being male
... and the ones that won't