Why there is a lack of transparency in US tenure-track job searches?
There are several factors at work here:


Remember that there are several thousand colleges and universities in the United States. A central database of such jobs would be significantly harder to publicize and organize.
Most universities in the US are private. They are under no such compulsion to post their jobs on any particular web site, unless constrained to do so by the funding sources that are supporting a particular position, or legal requirements to do so. (To my knowledge, there is no such requirement.)
The more deadlines and constraints you build into the cycle, the more pressure you put everyone under—applicants, recommendation writers, support staff, and departmental faculty involved in the search.


So I think there are multiple reasons not to publicize the results of a search, and I think most hiring committees would be reluctant to do so. There could be a better job done of announcing searches, certainly, and that would make things easier (but again, the places that currently advertise would likely complain about losing their business to a central source!).