The reason for this is a direct reaction to the abuses of nepotism, cronism, and other assorted officeplace "isms" that dominated government offices in the
18th, 19th, early 20th centuries.The sheer corruption in many govt. jobs in the past finally upset people enough that a system was devised to try to insulate govt. jobs from many of the problems found in any human organization.
This is not dissimilar to military life/rules if you think about it. Naturally the military the rules are more stringent (i.e. military rules are designed for order in the rnaks, obedience, etc.), but one side effect is to reduce many of the negative "isms" of day-to-day work (although there is still room for them if someone if determined).
Firing the useless is perfectly doable in civil service. However, it requires management to actually do their jobs, including that annoying documentation aspect.
This is the biggest reason they claim it is so hard. I think it is reasonable to give someone extra training and/or direction if they simply need help before just firing them (assuming typical job). But, if they really are incompetent (other), I think it is only right to force management to document everything (if nothing else, to defend agianst the inevitable lawsuit). If they aren't willing to do the documentation, obviously management has decided the worker is not a problem after all.