IQ can't measure intelligence because intelligence is qualitative, not quantitative. It's not like height or weight, it's like beauty. People can generally agree on it, but there's no concrete way to measure it.

IQ is useful, and only useful, in its "industrial" capacity, meaning where it's used statistically over large sample sizes. To explain this better, let's use the analogy of a Beauty Quotient (BQ). It would be calculated to give a higher value to large eyes, high cheekbones, etc. This could be used in aggregate to understand why so many female models come from the Czech Republic, but to apply it individually is meaningless. There are people with big eyes and high cheek bones who look like freaks of nature, and people with smaller eyes and lower cheekbones who are stunning. BQ doesn't exist because what it would measure is self-evident; you could just look at someone saying, "I have a BQ of 180! I'm gorgeous!" and agree (or laugh). IQ, however, measures something that can't be so quickly assessed, and so many grasp onto it to validate themselves. That's an abuse of the purpose and displays a lack of understanding of the difference between qualitative and quantitative, thus my jibe at losing 80 IQ points.