Tall is an absolute, objective measure of a real thing that exists. IQ is NOT a measure of a thing that exists.
Similar effects can be found throwing dice and ascribing the total to an IQ score, then telling parents, teachers what the poor kid's IQ is as though it were their height. It's insane.
I don't understand your concept of something "existing" or not. Do you believe that a "speed" exist ? That a "center of gravity" exists ? That a "temperature" exist ? People who say "IQ is what IQ tests measure" sound to me like they could say "temperature is what thermometers measure". temperature does not "exist" it is molecules movements all the way down, yet it is a meaningful/useful concept. Same goes for IQ or "g factor".
https://sites.google.com/site/7arosenthal/
Per: Rosenthal & Jacobson, 1966 study, which deserves wider appreciation?
I want to emphasise 1966 was when this was published so it isn't overlooked as we all read pretty fast with these things.
edit:
Found a link to the original paper:
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.488...
More recent criticism of the original:
https://thenewstatistics.com/itns/2018/04/03/weve-been-here-...