Really? I kind of enjoyed playing with Lego when I was younger. Sure, some form of challenge is always a good incentive to most people, but there is no reason to believe it must manifest itself as the current money for work system - working for social betterment, to ensure everyone has food on their table, a roof, etc, can be just as rewarding to some people. I mean, even the example you gave is kind of bad: if you bother looking, there are countless examples of stuff people did with the game that did not include any of the things you mentioned, and I'm sure it didn't stop them from enjoying doing them.