Because this is another thing your average American man in crisis does: he tries to go home, forgetting, momentarily, that he is the reason he left home in the first place, that the home is not his anymore, and that the crisis is him.
When I was a boy, I would read those postcards and know exactly why my father was doing what he was doing: he was taking a stab at greatness, that is, if greatness is simply another word for doing something different from what you were already doing--or maybe greatness is the thing we want to have so that other people will want to have us, or maybe greatness is merely the grail for our unhappy, striving selves, the thing we think we need but don't and can't get anyway.