Why is the war in Afghanistan is going on so long?

As the Taliban are not easy to eliminate, coupled with inefficient Afghan government / Internal Security Force which is akin to US puppet followed by some of the citizens are uneducated such that they supports Taliban, the whole operation is a failure, despite they have successfully removed Taliban government.

Well, Taliban right now uses guerrilla warfare, such kind of hit and run warfare was also used in China's civil war, Vietnam War & Korean War by the communist and they have the victory instead.