For two reasons. First of all IQ is defined as a normal distribution with a mean of 100 and a sd15. So by definition only 0.1% of the population can have an IQ of above 145 and this fraction is rapidly diminishing with additional standard deviations.
Secondl: the definition matches reality reasonably well because intelligence is a polygenetic trait and many small factors add up to a normal distribution.[0] It is statistically unlikely that natural processes will yield an IQ500 human. You would have to engineer one. Which leads us back to the AGI concern.