"Why doesn't the United Nations

do something about it?"
Iwas asked this question yesterday. "It" was the situation in Libya but it might just as well have been the situation anywhere in the region.
"It" might also be an earthquake, tsunami, flooding, a volcano erupting or any other disaster. There still seems to be a perception that this is the sort of situation that the United Nations deals with. It does not. Even UN "peacekeepers" are not what they appear to be.
The United Nations has no power at all. Only the member states have power. They can get together and do things but even then the United Nations has no power.
The United Nations Security Council and the "resolutions" which come from it are not worth anything at all unless the members who have voted for those things take action. Given that the permanent members of the Security Council can veto action as well it provides another excuse for people to do nothing.
Of course meddling in the internal affairs of another country is rarely to be recommended. In places where it perhaps should happen, like Somalia, the United Nations has proved remarkably ineffectual - and likely to remain so. Sending in peacekeepers from a disparate group of countries who have not worked together and who only have the right to fire in self-defence will do little to sort out the problems of any country in a state of chaos. United Nations peacekeepers can often scarcely keep the peace between themselves, let alone assist others to attain it. As a peacekeeping organisation the United Nations is in fact fairly useless.
There are member countries of the United Nations who also owe vast sums of money to the organisation. It wastes billions of dollars. It is corrupt.
Some of the International Years have raised awareness of issues like disability, water and literacy - to name a few. Education and health have benefitted - although not as much as they might have. There has been some excellent work done in preserving the "at-risk" culture and heritage of the world.
The United Nations can recommend sanctions against Libya but it cannot enforce them. It cannot prevent Iranian ships sailing through the Suez for so-called military exercises with Syria. It could not prevent the Taliban destroying thousand year old statues in Afghanistan. It cannot prevent human rights abuses in countless countries.
As an organisation it is of very little use and very little value but it is there and many people feel that the world is safer because of it. It may well do some good for that reason. Just do not expect it to do too much.