Because they are calibrated not to measure innate intelligence, but our own cultural norms. The canonical example might be, imagine you were lost in the Australian outback. Would you rather have Albert Einstein as your guide, or a skilled Aborigine? The Aborigine would likely get a low score on logic puzzles, but at highly skilled activities like way-finding and tracking game, he would beat Einstein hands down.

IQ doesn't measure intelligence, it measures artifacts of intelligence that appear in our culture.

Also, it's a mistake to think that IQ is static. There have been studies that show wild swings in IQ over time, especially in teenagers, and even changes in the collective IQ of a culture (the Flynn effect).

In addition, it seems likely that a person could study and improve their IQ score.

The counter-argument is that although IQ tests have flaws, they are still to some degree a predictor of success in life.