Good point. Ultimately this leads me to question the existence of some fixed quality of intelligence. People are growing, adapting, and learning through their lives, so a fixed number defining general intelligence is likely a moot concept.
On top of the prior point lies another major issue with any sort of "general intelligence" test: defining "general intelligence". Intelligence comes in many forms: linguistic, logical-mathematical, musical, visual-spatial, bodily-kinesthetic, interpersonal, intrapersonal, naturalistic, existential intelligence, and more. The IQ test does not test all forms of intelligence.
This being said, It is likely impossible to test all forms of intelligence in one test; and even if we could create this test, how would this test handle differently abled people. For example, a completely blind person would fail the visual intelligence portion every time (for obvious reasons).
This is a good point to bring up, but this correlation is still being debated: the causal connection between the IQ test and the correlation is unclear, and there is debate on whether the correlation is being constructed through bad data or analysis techniques. Because of this, no one can confidently claim whether IQ tests predicts good job performance, employment, etc.
https://pmc.ncbi.nlm.nih.gov/articles/PMC4557354/
[Skip to the conclusion at the end to get the tldr, since this is a long scientific publication]