are you honestly suggesting that a large part of the population can be trained to become research mathematicians?
Yes. The aptitude of higher abstract thinking varies greatly among different people and groups.
If you are really curious, check out the IQ test about pulling info from a sports column.
After that, your question becomes similar to asking why every short person can't be taught/trained to do well on a professional basketball team.
The number of active NBA players is in the hundreds and their careers are short. Education is a very effective weapon against multi generational poverty and the focus of international aid organizations for a reason. Someone from an impoverished background is much more likely to live a better life with more education than someone without.