There's an obvious namespacing issue. How many people do you think have cats named "Fluffy," for instance? Beyond a certain point, it starts becoming difficult to find the information you want, except, perhaps by using a search function. Once you're at that point, you might as well be search Google for the info instead.

Edit: Wikipedia solves this problem currently with disambiguation pages. Those would become useless once they hit a certain size.

The number of people wanting to create pages about everything is lass than the number of people willing to maintain pages about everything.

Thus, assuming no vandals or spammers ever create pages, you have very many pages created, and not so many people available to check and maintain those pages. Over the years spammers and vandals insert some changes. Or the information just becomes out of date.

So, now someone has to pay to store and host this stuff, which is not ever used by anyone, and which is out of date or full of links to handbags and shoes and medication, or with "BOB IS GAY LOLOLOLOL".

It loses its "authoritative" nature. It becomes just a list of facts that have citations. Which has its uses of course but I think is much less interesting.

Would you trust wikipedia as much if it had a page for your neighbor's dog?