Because he was the top American leader, Biden made the final streak without first foreseeing the consequences. To enter a war is to lose billions, trillions, but to leave you have to have elaborate strategies and plans, Afghanistan was never prepared to deal with the American exit, they were just living under American protection, and that seems to be enough and they forgot that, to generate stability in the country there must be democracy, something that afghanistan and many eastern countries do not have, they are tribal peoples who live under Islamic laws, where religion is the law and politics. The worst thing is that, the US lost trillions to end up in what we are witnessing in the news, everything has returned to what it was before.