The reason is very simple: it needs smart people to operate it, yet it rewards such people with careers so intensive and perhaps so interesting that they don't reproduce much. They are outcompeted by people who do not share their gifts for conscientiousness, affability and general competence.
We have new quantitative data which allows us to make some disconcerting predictions.
I think it was La Griffe du Lion who first wrote about smart fraction theory, observing that modern societies are complex networks of interdependencies which need a level of cognitive competence to make them work at all.
Basically, when you don't have enough smart, conscientious people the bureaucracies don't function, stuff doesn't get maintained, institutions break down or can't be created in the first place.
Garett Jones updated and popularised this argument in his book, Hive Mind. I reviewed it here.
La Griffe empirically set the smart fraction edge at (verbal) IQ = 106, but I'm going to be more conservative (perhaps AI will help?) and set it at 105. Let's look at four areas of the world, with differing average IQs and see what fraction of the population constitutes the smart fraction.
(Data here and here).
First China and Japan, with an average IQ of 105. Half the population counts as their smart fraction. We know what those societies can do.
(La Griffe would use ~100 for the North-East Asian verbal IQ here - his correlations show best fit of verbal IQ with capitalist success. Other races don't show a divergence between verbal IQ and general IQ, see La Griffe's article for more. I tend to think that all those engineers with their enhanced visuospatial abilities are doing wonders with the GNP.)Secondly, Western Europeans ('Caucasians') with average IQ 100. The smart fraction is the 37% of the population with IQ ≥ 105. These societies seem more dysfunctional than East Asia but still work most of the time.
Thirdly, the Middle-East with an average IQ of 85. Here we have countries like Egypt and Iraq, where only 9% of the population have an IQ ≥ 105. These are societies which don't function in a healthy way - corruption is endemic and innovation is poor to non-existent.
Finally we come to Africa, where the most optimistic estimated average IQ is 75. Here only about 2.3% of the population constitute the smart fraction. Given this tiny percentage, it's no surprise that advanced capitalist societies simply don't exist, or those previously built in region cannot be maintained.
The US Government Department of Labor Bureau of Labour Statistics publishes a detailed spreadsheet showing employed persons by detailed occupation, sex, race, and Hispanic or Latino ethnicity across 149 million people.
Ethnic/gender correlations with IQ/personality-type prerequisites of jobs are striking.
Much emotion would be spared if people simple browsed this table and thought about it.
It was the development of large-scale societies in the neolithic revolution which kicked off selection for the kinds of cognitive skills which are only required in complex societies.
It's taken ten thousand years to drive human intelligence up a couple of standard deviations (30 IQ points).
According to the latest, detailed study from Iceland, the fertility-reduction of our smart fraction is decreasing average IQ by 3 points per century: 15 points in 500 years if the trend continues.
At this rate, a mere one thousand years would be enough to wind us back to hunter-gather levels.
With fewer competent people being born, skill-shortages increase for the more complex roles. Standards are lowered while performance drops .. and drops.
Unless the right-hand side of the cognitive bell curve starts having large families as the norm, in a few centuries our western societies are going to look a lot like Egypt or Iraq today. After that it will get worse.
Trendwise, things don't look good: we're seeing a demographic implosion.
To which you can add the allele-frequency impact of uncontrolled immigration from that region.
'West Hunter' thinks we should worry about this more than stuff like asteroid impacts or climate change.
Self-induced social-dementia means you can't solve any problem.