I don't know, it mostly makes sense to me.

Intelligence is an evolutionarily adaptive trait, but it's not necessarily the most important evolutionarily adaptive trait.

You figure all the good traits have opportunity costs. Intelligence probably is reasonably 'expensive' in multiple ways -- tends to need a larger organism that can support a big brain, tends to be able to gather excess food/resources to support it, tends to need a longer lifespan to develop/function than some other traits, etc. Probably an organism needs to be past certain thresholds in mobility, defense, food gathering, etc. before intelligence becomes a 'good buy'.

I assume if life on earth was somehow left alone long enough a variety of highly intelligent species would develop; the reason that hasn't happened is that once you get a first (presumably?) species on that level, in a lot of ways they step outside of / alter the course of normal evolution* such that the balance of trait value is all shaken up again. For example, now for squirrels 'instincts that avoid being hit by cars' or 'ability to scavenge Cool Ranch Doritos'** might suddenly have high evolutionary values as traits, whereas a thousand years ago those traits would have had no value at all.


* It's reasonable to say that humans are still evolving, but the pressures/choices of humans in modern human society, from an evolutionary standpoint, are vastly different than they would be for humans in the wild.

** I swear I have seen many squirrels forage these out of garbage and eat them.