I asked why not and here is what I was told: "Every federal manager fires one lousy worker and no one ever fires a second." Smart managers don't even fire the first poor worker; they ask their mentors, are told "Don't bother," and take that sage advice.
Of course I asked why. The answers boiled down to: "Firing a poor worker is too punishing to the manager." As a social scientist who studies management, I wanted more detail, and got it.
In order to protect good workers against mean, vindictive bosses, federal personnel policy makes termination for cause so difficult that the process takes up to three years. Employees must be given multiple warnings - chances to improve their work - and they also have multiple appeal rights.
During those years the needs-to-be-terminated employee sits in the unit, occupying a position, doing no work because they are "working on their appeal," grousing about what an a**h*** the boss is, doing their level best to poison the morale of the unit.
During the up-to-three-years the termination process takes, managers spends endless time responding to appeals, going to appeal meetings, and trying to prove they aren't mean and vindictive. That's in addition to getting the remaining employees to do the unit's work, including the work the to-be-terminated employee should be doing but isn't because s/he is supposedly "working on the appeal" but mostly is just moping and complaining.
The federal personnel system is set up to protect employees, not to produce an effective workforce. This means there is a reasonable chance the attempt to terminate will fail and managers will be stuck forever with a sour, angry, marginally productive employee.
However, let's be optimistic and say managers succeed and terminate the employee. Now they can probably hire a new employee to replace the terminated person. If managers are lucky, the group's morale isn't poisoned and they can finally get back to doing what FedGov wants done, plus training a new employee.
For two-three years managers have spent all their time on the termination process instead of on projects which would make them more promotable. Although they've done us taxpayers a favor by ousting a rotten employee, managers normally end up suffering for it. If you want proof, go see this USAToday article.
Federal managers who have poor employees can ignore them, and many do just that, or try to get rid of them by means other than firing them. Believe it or not, one way to get rid of losers is to write them glowing recommendations in an attempt to foist them on another agency. It was called "turkey outplacement;" I saw multiple examples of this happening.