It really depends. Most times you just need the money. Sometimes you have nothing better to do. Many times you just keep doing it because you are used to doing it. A few times is your dream job and you just can't stop doing it. Work is more than economics, is not a function of money. Work is action. My father is a 73 'retired' professional and scholar, now serving as a congressman, and already thinking what to do next. Action is life and you should never stop it: work til you drop.

Another question would be: why do we keep working so hard for money when technology could already solve many of our needs?