Why the war in Afghanistan is still on-going is the $20,000 question, isn't it?
As to the other answerers who seem to think that Afghanistan does not produce oil, they are mistaken.
According to CIA-The World Factbook, Afghanistan's natural resources list includes: " natural gas, petroleum, coal, copper, chromite, talc, barites, sulfur, lead, zinc, iron ore, salt, precious and semiprecious stones " (ref #1)
Also, according to another site: "Natural gas fields are scattered throughout much of Afghanistan. Recent analysis by the United States Geological Survey has indicated significant unexploited oil reserves in the north as well." (ref#2)
And one more comment from the Institute for Afghan Studies states, "In February 1998, the Taliban announced plans to revive the Afghan National Oil Company, which was abolished by the Soviet Union after it invaded Afghanistan in 1979. Soviet estimates from the late 1970s placed Afghanistan's proven and probable oil and condensate reserves at 95 million barrels." (ref#3)
People, Thar's Oil in Them Thar Hills.....