Because of the construction of the tests. As you go to higher points, each point represents fewer and fewer correctly answered questions. Matrix IQ tests can be mechanically generated by combining simple rules, and they show the same bell curve norms despite what look only like linear increases in number of rules or complexity.

And the Benbow and Terman studies and others do show practical impact, but they don't show a linear impact where each IQ point is as valuable as the previous, and they certainly do not show an increasing marginal returns where the next IQ point gives greater benefits than before!