Well, we put them out there when the client wants/allows us to. However, they often do not and there are a few reasons we may treat the employers names as confidential:

1. They may request that we keep their name strictly confidential. This might be because they are replacing an existing employee (that they are promoting or letting go).
2. They may be hiring for a critical position and they don't want to attract attention of their competitors or the media.
3. They may not want to be inundated with unsolicited candidate and recruiter communication.
4. We really like keeping secrets. It makes us feel important :).

Also, as a candidate, we treat your name confidentially (if you so request). There are times when you may not want your name to "get out there" because maybe you don't want your current employer to know you are looking for a new job.