I'm not sure theres such a thing as smart people to begin with. Plenty of people know their own domain very well, but it seems only a few domains are arbitrarily chosen as intelligent ones. Seeing as we're still struggling to define what intelligence even is, I don't see how we can start labelling people as intelligent or not.

At that point, the question is just "why aren't people happier". I doubt theres any one answer, but a lot of the responses in this thread seem to point to some of the possible reasons.