Yes, it's often the time to fork and take over. But it's a big project in itself, not everyone wants to be the new developer-in-chief.

Sometimes, a project can stay at the borderline of death and alive - there still can be minimum maintenance by a not-so-active developer, major distributions still have packages for it, etc. I recently sent a patch to a project - it took 11 months, but it was eventually merged when a developer with commit access finally noticed it. There was no motivation for me to fork it since all I wanted to merge was a simple fix, I was not planning to take over the project.

Some zombie projects are far too large for an individual to consider maintaining.

OpenOffice is a perfect example of such a project.