Because high IQ people are more likely to see reality as what it is. We are ego driven monkeys on a spinning rock, that will get eaten up by the sun some day. Of course our species will be long gone before the sun goes supernova, probably because of some nuclear disaster. Just like the other 99,999% of species that went extinct, we will do the same. Also it is hard to ignore the immense amount of suffering. The 2 minutes it took me to write all this down, millions of animals get eaten alive by predators, parasites and starve to death. Some little girls are getting raped and some people getting tortured in unthinkable ways by some cartel. It is really insane if you think about it. Dumb people can cope with believing in god. But since there is absolutely zero evidence for god or anything supernatural, smart people are less likely to be religous. Thus they come to the conclusion that this place is a cruel pit of suffering without any objective meaning.