Because high IQ does not necessarily mean development. Read the book why nations fail.
Countries are poor because in poor countries all the power is in the hands of a few ruling elite who only care about enriching themselves. Dictatorial regimes can develop only if the elites themselves want the country to develop (like china or Singapore)
So you could have a country with an average IQ of 150+ but a small group of powerful people control everything that country will remain a shithole for everyone except the elite.
And that's the reason why north Korea is so fucked but not south despite being of the same IQ