The United States won't finish the job in Afghanistan because the job that it should finish and the job in President Obama's mind are two different things.
The job of the United States and NATO in Afghanistan is to defeat the Taliban and Al-Qaeda and remove their ability to sow terror in the U.S. and other western nations. The job in Obama's mind is to get the United States out of the country as soon as he possibly can with the least amount of political damage to himself.
President Obama is a politician in the worst sense of the word and doesn't have the mindset to bring the war in Afghanistan to a successful conclusion. That would require him to be resolute and strong with a clear understanding of why the U.S. and NATO are there in the first place. It would require him to make a long term military commitment on the part of the United States regardless of what other NATO nations do, and regardless of the political consequences at home. He would have to place his anti war mentality aside, exercise real leadership and vision and be a true war time President. There is nothing in his record that he will ever be willing or able to do any of this and that is why the Taliban and Al-Qaeda won't be defeated in Afghanistan or anywhere else as long as he is in office.