A year ago
Teams from Africa (except from North Africa), usually aren't that good in tactics and I guess that you can't change/improve those players too much for a top European level, except for some rare exceptions.
Teams and players are far behind world class European level in terms of tactics, movement, positioning, discipline.
Regarding Europe, all teams have scouts in Europe and it is hard to find a cheap, hidden gem.
So, African players usually have some tactical problems.
Asian players are rarely on a World class level for Europe.
European kids are scouted by everyone.
And all that is left in terms of cheaper gems is South America, especially Brasil, Argentina, Uruguay who have good football schools, good talents, they are tactically good.
Further, after Mina and Marlon, it was easy to predict that we will repeat that recipe with buying good, young Cbs in every season and:
1. Either keep them, if they are world class
2. Or resell them to EPL rich teams
I hope that my post doesn't sound racist...
They can't retain them forever, that is true, but there are other clubs, not only Barça, that are interested in their gems.