Speaking for myself and Collaboraid, we haven't built any CMS at all. We haven't had any clients yet who wanted CMS functionality (badly enough to pay us to do it), and for our own web site, we just use SSH and Emacs.
I think most of the other companies are either the same way, or the CMS's they've built are too client-specific to be of any use, or they just haven't really bothered to make it publicly available.
I don't think there are short-term commercial interests at play here. But I could be wrong, of course.