Because they all believe that a) their own financial success proves they are intellectually superior, because they believe capitalism is a perfect meritocracy, b) that intellectual superiority is 100% heritable, so it will definitely be passed down to the children and therefore c) it is their duty to improve humanity by making as many genetically superior children as possible.
It's basically eugenics. They all believe in it because they all listen to Mencius Moldbug.