Shouldn’t the real question be “why aren’t more people happy”. Lack of happiness isn’t relegated only to “smart people”. My observation: managing stress related to ever growing amount of responsibilities (as comes with age), finances, and spouse can have a disproportionate impact on one’s happiness. None of which are tied to intelligence.
If you use your smarts to conclude that having a spouse will negatively affect your happiness, don't have one.
A very intelligent person should be able to figure out a way to improve their finances.
The article makes the distinction between well-defined problems (eg. increase my monthly income) and poorly defined problems (eg. find a spouse that matches you for life), and clearly suggests that intelligence helps with one but not the other.