This is a question that is answered for you if you re-read Chapter Ten of this amazing book. It is quite clear that, now that Hassan and Ali have left, and that the Taliban have seized power, Afghanistan is no place to stay for anyone who has the resources to leave. The danger and autocracy of the Taliban regime, when Baba was never that "religious" anyway, would have been terrible to him. Therefore, Chapter Ten charts how they are smuggled out of the country. This of course is no easy process. They face drunk Russian soldiers who threaten to rape one of the women until Baba stands up for her and then a superior officer comes and lets them go. They face other technical difficulties and face living in a basement for over a week with rats. Eventually they leave smuggled out of the country in a fuel tank of a fuel truck which does not allow the refugees enough oxygen. When they finally leave the tank, they realise how the son of Kamal, another refugee with them, had died during the journey. In a fit of grief, he steals a gun and shoots himself.
What is important to note, especially for us in the West who have not experienced such oppression as Afghanistan experienced under the Taliban, is the desperate nature of such refugees who are willing to do anything to leave their home country. This should make us think very carefully about how we welcome such political refugees into our country.