Because it is a chance to work with other people and broaden the perspectives of wikipedians rather than just saying "fuck it", and doing your own thing.

Working with people who disagree with you is a pita, it's not something that most people are willing to do, and it can take a seemingly wasteful amount of time. Getting along with others is the real challenge, any yahoo can go make something and convince a few other yahoos that it's the best way.