The reason for this is pretty simple: you get free public education in these poor countries, but only if you pass the entrance examinations. For a lot of families this is the only way for their kids to get out of poverty. Moreover because of the way the school system is structured, your kid could easily fall behind at age 7 and never recover. The reason for this is because of stratified classes, where they group kids…
By contrast, in countries that have been industrialized for over a century, the preceding several generations found that they could relatively easily find low- to moderate-skill factory jobs that secured a reasonably prosperous lifestyle.
There are also industrialized countries with stratified school systems, and I see that some of them fall toward the bottom of the graph.