Werewolf,
My guess would be that it's because the idea behind government is that it's (supposed to be) a way of moderating the diverse elements of a society into something like a consensus, and then taking action under the common beliefs that most of the people in the society hold.
Thus, if (for example) most of the people in the society believe that theft is wrong, and that the proper punishment for it involves restitution and being confined for ten years to Think About What You've Done, then that's what the government will impose on the thief.
The reason that government frowns on vigilantism and the reason that society as a whole frowns upon it are divergent. Government frowns upon it mostly when government has become so large and ineffective that its only remaining goal is to be self-supportive. Vigilantism and other forms of citizens taking the law into their own hands are not as opposed by agents of government when those agents are trying to serve their actual purpose but just don't have the resources to do it. (Outlawry was an example of this; declare a criminal an outlaw, meaning that he was not under the protection of law, thus making him perfectly legal to kill, and the citizenry takes care of it).
Society, on the other hand, frowns upon vigilantism because a vigilante is an individual, not statisitically representative of the consensus of the society. So if a vigilante believes that the penalty for theft should be evisceration, he's carrying out the law as he sees it, but he's imposing a harsher penalty than the society supports, and so he's viewed as an extremist.
It is almost a de facto thing that a vigilante will be viewed as an extremist by society, because the views of any one individual will always be more polarized and extreme than the collective views of a society that comprises many individuals with differing views. And only a person who feels like "not enough is being done" will resort to vigilantism. Therefore, a vigilante's actions will always be harsher than the society at large supports.
Now. Legitimate government comes from the consent of the governed, so if the government is failing at its job, the consent of the governed should be removed, and given to a new plan or body of people who can do a better job. And in a society where the people who understood that fact were the majority, it would happen that way.
In a society that gets angry that they might have to miss an episode of American Idol because there's a special news report about "Some political thing. I hate politics!", well, we get what we have now: bread and circuses.
Just my thoughts, worth what you paid for them.
-BP