You would think after a century of research we would have this figured out. But I still see debates about whether IQ tests capture real intelligence, whether g is a meaningful construct, whether different tests give you different scores, and whether the whole framework is even valid. If intelligence is a real thing that exists in the brain, why is it so difficult to measure cleanly and why do experts still disagree about how to do it?
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The core difficulty is that intelligence is a latent construct, meaning you cannot observe it directly the way you can measure height or blood pressure. You can only infer it from performance on tasks that require it. That introduces noise at every step: the tasks sample only a slice of cognitive demands, performance varies with motivation and anxiety and fatigue, and different batteries emphasize different abilities. The disagreements among experts are mostly about the architecture of intelligence rather than whether it exists. Whether g is best modeled as a single factor or a hierarchy of correlated abilities, whether processing speed is a cause or a correlate of g, these are genuine open questions. But the practical measurement tools converge enough that scores from well designed tests correlate strongly with each other and predict the same outcomes. The difficulty is real but it has not prevented the field from producing some of the most replicable findings in all of psychology.
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Difficult to measure perfectly is not the same as impossible to measure usefully. We cannot measure pain directly either but we still make meaningful clinical decisions from imperfect proxies. IQ tests are among the most validated instruments in social science despite their limitations.