Because we're depressed at being surrounded by stupidity.

Or at least I am; I won't speak for everyone else.

Speaking only for myself: you can't be tortured by thoughts you're not having, and I, sadly, am having thoughts.

Why should being smart make you happy?

I think the essay has the implicit assumption that if you're 'better' at doing things, you'll end up happier. Since intelligent people aren't happier, the conclusion becomes that intelligence is mostly only relevant to a subset of well-defined abstract problems that don't represent the real messy problems of life.

I think the reality of human happiness is that being in better material circumstances, or being better at solving problems practical or not, doesn't necessarily improve long term happiness.

Adaption to circumstances, the 'hedonic treadmill', means most people aren't going to obtain some very high level of happiness and sustain it indefinitely, regardless of station.

Maybe because they are not that smart? :)

"Ignorance is bliss" - Cypher

Intelligence is a slippery term, as plenty of smart folks have made dumb decisions. That said, I’ve noticed that many people who don’t understand why their choices are subpar tend to blame others, i.e. life happens to them. Someone more aware might internalize their failures. Plenty of mutations of this mechanism.