Why are we in Afghanistan again?

I understand fully why we went into Afghanistan. We were attacked, the masterminds were in Afghanistan gloating and threatening to kill more people, we had trained to instigate and support an insurgent driven guerilla war for decades using Afghanistan as our primary model, we had a huge standing military that consumes the lion’s share of our budget. We had better be going into Afghanistan after 9-11.

But now bin Laden is dead (What a great time that would have been to leave, huh?) , most of Al Qaeda is gone, the Taliban has been devastated — even if they claw their way back to power in some regions, it will be a long, long time before they forget what happened when they let a crazed religious nutbag come from outside and drag them into a conflict with the US. We Have Won. So why the hell are we still in Afghanistan?