Some of the reasons why it could be considered part of South asia would be its reliance on Pakistan for international trade as one of the easiest entrance points and its cultural distinctness (linguistic and historical ie not part of the Soviet Union) compared to the rest of Central Asia. read more
afghanistan is far from being a middle eastern country because the majority of the population are south asian but there’s a minority that are central asian. so afghanistan could be considered as both central and south asian. read more
So if Pakistan is included in the South Asian region, Afghanistan has to be a part of the South Asian region as well. Since its in the middle of the ASIAN - EUROPEAN continent. It is just a matter of which direction you are going from west to east or south to north. read more