Afghanistan is a big country in terms of area. You need to compare it with small countries in Europe who are considered as giants based on economy and military power like France and England.
Regarding no access to sea and all. Afghanistan sets next to China, Russia and is also a gate way for Europe. Reason why it is called 'Heart of Asia' . China's one belt one road is passing through this country, additionally North south corridor, and other connectivity are developing.
This country has mines, good weather to grow fruits, vegetables, cattle farming etc..etc..
Since the population density is less compared to resources, they can develop faster if funds are induced into their economy.