They laugh because Wikipedia has turned itself into an international laughingstock. This goes well beyond simple inaccuracies in articles. Due to its open editing policies, it can be made into a game, a revenge platform, a libel platform, a MySpace-type place, or practically anything else you can do with a website.
While it is true that these things are against Wikipedia policy and doing them may get you blocked or banned, it may take the admins awhile to get around to you. There are only about 1,700 admins on Wikipedia, many of whom are inactive, and changes come to Wikipedia every few milliseconds. If you get caught, you can start over again with a new account. Many have done just that.
Accordingly, it is more accurate to describe Wikipedia as a "MMORPG", rather than an "encyclopedia". Besides which, people who regularly edit Wikipedia can get up to some very silly behavior (click on the first link below; the second link is a glossary of terms used on Wikipedia).
In sum, Wikipedia is a lot like Camelot: better avoided as it is a silly place.