The matter of wars being fought in different areas of the world, as well as continuing U.S. Military operations in the Middle East, makes this a more subjective question. Obviously, there are absolutely objective reasons why U.S. Armed Forces remain in battle, but there are also public opinion -- subjective -- reasons, and probably many backstage facts that are not publicly known.
For example: the U.S. invasion of Iraq in 2003 resulted in first the toppling of existing government, and then a continued occupation by troops while the region attempted to stabilize -- it was a long process. As of today, combat troops have been fully withdrawn from Iraq (08-18-2011) while multinational support forces -- including U.S. soldiers -- remain in the region to assist the Iraqi government. As you can see, this is a case where the official war -- Operation Iraqi Freedom -- has been declared finished, but non-combat personnel remain. It's a tricky area.
As far as Afghanistan goes, there are more details about the various operations involved than can be summed up in a discussion post. The current status is that many supporting nations are withdrawing or have withdrawn their own troops -- Canada, for example, has completely withdrawn -- and while President Obama has made several references to withdrawal, there are still many troops on the ground and the fighting continues daily. There is a case to be made on either side -- you can argue that we should withdraw completely and let them sort out their own issues -- you can argue that our presence is keeping Al Quaeda and the Taliban busy so they can't launch as many attacks on outside targets -- but in the end, we as civilians can only make our opinions known and hope somebody is listening.
As far as Libya is concerned, Allied forces there have been extended for another 90 days; we could withdraw now, but our track record suggests that we will have troops on the ground there for a long time.
To straight-up blame Obama for failing to withdraw troops from the various hot-spots around the world is to ignore the larger issues, as well as assuming that we know everything our government knows. Remember, Obama also said he would close the Guantanamo Bay detention camp, and as of today has not started a process -- my subjective view: it was a campaign promise and after inauguration he was given new information that changed his mind, information that, as a candidate, he could not have known. I can apply this supposition to almost any change in policy.
There are more reasons for political action in heaven and earth than are known in any single, subjective philosophy.