They're needed by their job, and their friends, society. For a lot of people, when they retire, they don't really have anyone to talk to anymore. Maybe they don't have many personal friends. Maybe they don't have a strong marriage, or their spouse passed, or they never married. Maybe their children have moved away, and are so busy with their exciting lives, it seems they never have time for anything more than the obligatory weekly skype session.

This can make your health decline.