Because intelligence is a quality, not a quantity. That makes it like beauty and flavor, not height or weight. You can't put a point scale on it. Chocolate cannot be 7 better than vanilla. A beautiful person is not the one with the biggest lips and the largest cheekbones.

We we can do is to put general parameters around it, such as "beautiful people tend to be taller." In the same way, scoring generally well (over 120 or 130) on an IQ test is like saying you're tall. It's actually meaningless as to how smart you are, but they share some of the same parameters, so they will correlate a bit more.

Maybe it is like money and its relation to happiness. It can help up to a point to increase your happiness. But, once you can cover your monthly expenses, it doesn't help your happiness. After which, stuff like friendship come into play. So, after 130 maybe, stuff like your choice of academic supervisor or resources to do research come into play in determine whether you have a shot at the nobel prize.