When OS X came out (both the Mac OS X Server 1.0 and Mac OS X 10.0 varieties), the UI might have looked different, but you were basically running OPENSTEP 5 with a new coat of paint. Haiku could do a new theme fairly easily at this point, and there are already quite a few touch-ups of the new-slab-of-paint variety (e.g., subpixel antialiasing, support for tiled windows, various improvements to OpenTracker).
The other thing that OS X brought to the table were new APIs, but most of these APIs weren't really user-visible per se. (E.g., QuickTime was a genuinely new addition to OPENSTEP, but the end-user would just see it as more videos played now, and they played with lower CPU usage.) Here, too, Haiku has delivered: it has new APIs for component layout, new APIs for end-user notifications, and more.
So your comparison to what Apple did with OS X is actually quite apropos. Be just hasn't felt a need to overhaul the UI as radically as Apple did in going from OPENSTEP to OS X--in large part because, while some of those changes were functional, many of them were more about making a statement than being genuinely easier-to-use. (Note how much OS X 10.6 and 10.7 have largely reverted to a kind of brushed-up version of the old Platinum interface: no more pinstripes, flat grey buttons in most cases, flat grey title bars, etc.) So I think they're doing exactly what they ought to be doing.
I assume they don't exacly have a huge community of active devs. combine that with the fact that user interface design is hard and needs experienced people working on it and you got the reason for cloning a perfectly functional UI instead of completely messing up while innovating a revolutionary new one.
This is pure conjecture, of course :)
