Yes, we all know what IQ is. This cannot improve in a species if it isn't a desirable trait, simple as.
Knowledge burnings, and subsequent culture, makes an environment that incentivizes more brutish behaviour. This passes on with mate selection, determined heavily by the environment/culture, and the species IQ improvement slows. Higher IQ hasn't been a very desirable trait for most of history.
Everything can be boiled down to cause and effect since nothing is exempt from this logic. Denying how and why a species evolved, a form of history, is like arguing cause and effect isn't real.