If the UX people you've been working with haven't been doing post-implementation statistics and analytical measurement leading to refinement, then they are not UX people. They are visual or interaction designers playing at being UX people.

Unfortunately, the hard truth of it is that most "UX people" are in environments where the post-implementation cycle is ignored. For as many companies out there that claim to embrace the "UX process," I'd say that well over 80% of them don't actually embrace it beyond feature release.