I wouldn’t call it magic, but what makes FSIQ tests special is that they’re specifically crafted to estimate g. To your point, anything that involves intelligence (SAT, ACT, GRE, random trivia quizzes, tying your shoes) will positively correlate with g even if only weakly, but the correlations between g factor scores and full-scale IQ scores from the WAIS have been found to be >0.95, according to the same Wikipedia page you linked in a previous reply to me. Like both of us mentioned in previous replies, using imperfect proxy measures would necessitate multiplying your sample size because of diluted p-values and effect sizes, along with selecting for many things that are not intelligence. There are more details about this in my reply to gwern’s reply to me.
I wouldn’t call it magic, but what makes FSIQ tests special is that they’re specifically crafted to estimate g. To your point, anything that involves intelligence (SAT, ACT, GRE, random trivia quizzes, tying your shoes) will positively correlate with g even if only weakly, but the correlations between g factor scores and full-scale IQ scores from the WAIS have been found to be >0.95, according to the same Wikipedia page you linked in a previous reply to me. Like both of us mentioned in previous replies, using imperfect proxy measures would necessitate multiplying your sample size because of diluted p-values and effect sizes, along with selecting for many things that are not intelligence. There are more details about this in my reply to gwern’s reply to me.