Because the US government is a) strong and b) doesn't think there's one. Neither of those is true of Afghanistan. The Taliban is quite capable of at least hold

Because the US government is a) strong and b) doesn't think there's one. Neither of those is true of Afghanistan. The Taliban is quite capable of at least holding on, and the non-Taliban Afghani government isn't opposed to US support. As evidence of this, President Ghani's reaction was to promise check and balances to reduce casualties, not kick the US out of the country.