Because contributions from new users are immediately reviewed by legions of volunteer cops who are eager to revert vandalism, and most wannabe vandals don’t have the sense to make a couple legitimate edits before vandalizing.

Can you imagine any other crowdsourced site who was willing to subject new users to so much scrutiny, and err on the side of deleting contributions?

Certainly that's not a great way to make money. Not if you're depending on people to spend a lot of time seeking new content (and be shown ads).