Because IQ tests test how intelligent you are! That is a fact, a pure, 100%, absolute fact. IQ tests are the best and most accurate way we have of determining intelligence. Basically it is a measure of raw brain speed. The faster the brain, the higher the IQ. It is also a measure of how big your brain is. The bigger the brain, the higher the IQ. People with high IQ’s have better short and long term memory. That can store more things in memory and retrieve them faster. They are better at abstract thinking.
In a word, they are simply more intelligent.
We could reword your question like this, “Why do people speak so highly of human intelligence?”
Doesn’t that sound silly? Don’t you think human intelligence is a pretty important thing? Are you prepared to argue that human intelligence has no meaning at all for human beings? What sort of an argument is that? Why would you want to argue such a thing?