How can this be? Simple. Anyone can edit a page, and all the recent edits are saved. So if some irresponsible individual ruins a valuable page, it can be quickly restored by anyone who knows how (it's not hard; click on the "View other revisions" link at the bottom of the page). Moreover, the irresponsible individuals know this. So they have little incentive to try to wreak havoc.
We might end up with people posting silly quotes, but that's to be expected...
"Share your knowledge. It is a way to achieve immortality." -The Dalai Lama