Evidence?
This is partially because they're designed to behave that way. There is not a direct causation between your score on a test and your quality of life.
Also, how many people are actually taking IQ tests multiple times in their life? Who even takes them once unless they're being studied for a mental health issue?
> Health issues, mental problems, stress, tiredness, drugs or alcohol can all influence your score, mostly in a negative way.
… and this is a counterpoint to the one above, because removing negative contributions is the same thing as improving your test score.