My experience as well is that most managers are not calculating/forward looking enough to do that (start looking for a replacement). Most are plugging holes in the boat as they go along. If they want someone to stay, they just avoided a headache of replacing them and are not giving it that much thought.

Also, at any decent size company. Firing someone outside of large scale layoffs is also a huge process. They won't just out of the blue fire you without a series of programmatic steps.