Why do some smart people lack common sense?

Because smart people can overthink things:

Intelligent people, however, have a tendency to overapply their analytical and logical reasoning abilities ...and, as a result, get things wrong. In other words, ...intelligent people lack common sense because their general intelligence overrides it.

Even more interesting is that IQ improves your side of the argument, but not your understanding of the other side:

...high IQ improves people’s ability to defend their own position but doesn’t have any effect on ability to better understand the other side’s position. So smart people are better at arguing their own views but they’re not necessarily better at understanding where others are coming from.