I feel like you're saying ignorance is bliss.
That might be right sometimes but it doesn't really have much to do with intelligence and IQ, as far as I know.
And since you mentioned IQ, one issue is that people with very low IQ often struggle a lot with everyday things, and that can lead to a lot of frustration, anxiety, depression, etc... Statistically speaking of course, I'm not talking about individuals since everyone is different.
Society can actually be pretty complicated and it's not a good feeling to be left behind, think about everything you need to do just to get by... Stuff like finishing high school, finding a job, paying taxes & doing other bureaucratic things, acquiring some sort of skill, building meaningful relationships, etc...