My intuition is that "smart" is a vague word that means a lot of things, but almost all of those interpretations are generally seen as a positive and respectable. The idea of being respected is inherently appealing, so if we entirely conflate the colloquial meanings with a very specific meaning that can be measured accurately on a linear scale, well then you can just show people your good number and take a shortcut to being revered without having to actually behave in an observably respectable way in front of other people.
A person taking an iq test has experience with claims of being smart being met with skepticism, so the next idea is that a third party would help clear up that misunderstanding. They're not paying to be told they're smart, they're paying for the certificate from a third party to back them up.
My guess is that overlooking the obvious issues is more about desperation than anything else. No one calls someone intelligent to convey that they can score high on a specific test that measures nothing meaningful. It also should be very natural to ask whether other people might find reason to doubt the value of a certificate. Not doing any investigation into these thoughts is pretty fucking stupid, but stupid to the point where I think there has to be a certain level of desperation to not see them at all.
I'm reading what you're writing as saying IQ tests should not be taken seriously but it also sounds like you're disagreeing with me for writing that I think IQ tests are a garbage concept that someone would be inclined to buy into if they're overly insecure and want a shortcut to claim that they're "smart". What did I write that you actually disagree with?
I was replying to a comment wondering how people can take them seriously and I was trying to imagine what could lead a person to entirely avoid looking at the very obvious reasons why iq tests should not be treated seriously. It feels like you're condescending to me while holding the same opinion I have.