See, from the outside, Wikipedia is an encyclopedia. And there's a community there to write an encyclopedia. Sensible, no?
Actually, the community comes first and the encyclopedia is a side-effect. The community functions very politically, and very much like a game as well. The original intention might have been an encyclopedia with the minimum number of rules necessary to make that successful, but ultimately, the rules and red tape turned into the fun part of the game, and the encyclopedia-writing is just the grinding bit.
If all you wanted was a huge encyclopedia unlimited by the constraints of physical book size, you'd have a deletion policy like this: "each statement of fact has to be cited from a reliable source, every article has to be composed entirely of statements of fact, any other content should be deleted". So if there are no reliably-sourced statements of fact about a given subject, you don't get an article about them. If I started a Wikipedia article about my neighbor's cat and wrote a lot of outrageous claims, they would all be deleted and we'd be left with a blank article, which would probably be deleted as a relatively boring matter of procedure.
But what fun is that, when we can have a concept like "notability"? Ah! Suddenly, instead of all that boring shit about citing facts and removing uncited content, and just keeping whatever reliable information we have about something, we can have a massive PvE game where you get to find articles about subjects you don't care about and declare "this isn't notable!", and then you can lawyer a "consensus" of other players into agreeing with you. Or if another player disagrees, suddenly you have a massive PvP battle! Fun! Well, at least for a certain type of person who enjoys lawyering and gets off on will-to-power stuff that motivates one to try and delete articles from an encyclopedia, which is exactly the type of person that gravitates towards the project. And--believe it or not--these kinds of highly-social games, and one's success in them on Wikipedia, are a major factor in getting elected "administrator", at which point your duties now entail judging these lawyering games and actually deleting the articles!