Because Zambia is poor.
To become a strong country, find a number of smaller countries and start killing their populations until they let you extract their resources and ship them back to your country. Eventually, leave the responsibility for killing people to local representatives that you give a bunch of guns and a tiny cut of the resource profits to. If the locals rebel, and look like they will be successful, find the strongest rebel and offer him the same deal. If he refuses, kill him, and go to the next strongest rebel.
Leave $1 in the country for every $100 you take out of it, and since you're leaving it with the elites that you created, they're just going to spend it on imports anyway.
Isn't that what the global south is for?