Also, in order to test IQ in the upper range, you need items that are difficult enough to be informative in the upper range, e.g. items where people with 130 IQ would usually answer wrong but 150 IQ would usually answer right. But such items would be wasted on the vast majority of test-takers who would get them wrong due to being lower than 130ish IQ. So statistically you get very little value from improving their accuracy in this range.
Also, in order to test IQ in the upper range, you need items that are difficult enough to be informative in the upper range, e.g. items where people with 130 IQ would usually answer wrong but 150 IQ would usually answer right. But such items would be wasted on the vast majority of test-takers who would get them wrong due to being lower than 130ish IQ. So statistically you get very little value from improving their accuracy in this range.