For a straightforward reason, we have failed to subjugate our systems of humanity to the freedom of freedom that would otherwise challenge, reassign, and renew evolutionary merit continually, and allow for optimal mobility of inequality.
Inequality is a fact of life, and equality is an oxymoron wandering down the halls of cheap governance and cheap populism. Yes, go ahead and feast on that statement for a second. Equality will never exist, and does not exist anywhere in nature – and should not.
Evolution depends on the value of our meaningful differences, not on our stifling commonalities. And the value of our differences is what expands the fringe of human capacity and ingenuity — the reason why I often refer to evolution as a fractal.
Now, the severe problem with the sharp contrast you correctly identify (for example 5 million children die before the age of 5 every year), is that our systems of humanity (economic systems, financial systems, political systems, education systems, social systems, etc.) not only harbor oligarchic inequality but worse proliferate oligarchic disparities. A pay-to-play of sorts with narrow-minded humanitarian consequences.
For our man-made systems are all based on the stronghold of an oligarchic monism of freedom at its foundation. Quite the opposite of the freedom of freedom we owe ourselves. Such oligarchic and unchallenged status quo, therefore, allows for very little mobility of inequality, and thus significantly restricts a meritocracy and the renewability a system promising to be free would otherwise yield.
So, the reason why the bell-curve of merit in our society is hugely unevenly distributed is that we have failed to subjugate our systems of humanity to the freedom of freedom that would challenge, reassign and renew evolutionary merit continually.