I think the lack of imitation is because people don't actually like OSX UI, other than people who are simply accustomed to it, or who like the idea or the fashionable commodity luxury appeal of Apple products. If people actually liked it for qualities divorced from brand or narrow experience, those of that group who had deeply held FOSS beliefs would have had any number of widely used workable imitations by now.

Apple UI is bad. The belief that it is good is largely religious and a result of branding.