Per Wikipedia, most IQ tests have a highest-reported standard score of 160 [0]; that's also consistent with the highest range for Stanford-Binet 5th Edition and Kaufman Assessment Battery for Children topping out at 160 (the SB5 top range starts at 145.)
For example, one serious official test I did included a timed test where you had to decode a string of symbols to text based on some key. I ended up being fast enough at decoding that I was effectively limited by how fast I could scribble down the decoded texts and turn the page back to the code, not by the mental calculations. I imagine others would have had similar issues, making this particular test at best capable of indicating a wide possible range of mental ability, a range which should be particularly inaccurate around the higher ranges as the ratio mentalTime/mechanicalTime decreases.
There also just aren't enough high IQ people to really effectively study the accuracy of these tests above those levels. There are no good baselines to compare these people and no real ways of checking whether the differences actually matter in any meaningful way.
Afaik, that's not a “designed to be usable by everyone” thing; there's test-retest consistency gets progressively lower beyond that point on IQ tests, period.
> There also just aren't enough high IQ people to really effectively study the accuracy of these tests above those levels.
If that was true, we wouldn't have reliable information that accuracy breaks down in that area, which we do.
We can study the accuracy in that range, we just haven't found a way to fix it.