America gave Afghanistan weapons when it withdrew on purpose. It wasn't incompetence. America is also currently massively funding Afghanistan through Qatar and other means. You can look up the list of mega projects and infrastructure Afghanistan is doing but this canal should be sufficient: https://www.youtube.com/watch?v=n6q_e94Ixb8.

But why?

Are they going to have the Afghans invade Iran and link up with ISIS elements in Syria and Iraq? Maybe also destabilize countries near Russia such Turkmenistan, Tajikistan and Kazakhstan?

Whatever the end goal is, it's not going to be pretty and it probably pushes the establishment of greater Israel project.