Because not everything is notable. Wikipedia articles are supposed to be encyclopedic. Why should the article that some local band creates about themselves be allowed? Aside from being shameless promotion, they're not notable in a larger context. The same is true of products and people. Actually with articles on people, you also have an issue with libel.
One problem with notability--and there's no easy answer--is that notability has context. To take an example that's relevant to this audience, there are developers and others involved with open source who are considered "notable" within open source communities who have never been mentioned in mainstream media. And certainly not printed media. But they're mentioned in blogs relevant to a narrow audience.
Similar situations happen with respect to geography. Maybe someone has appeared a number of times in a local newspaper and may even be considered notable on a local basis for some activity. Maybe they're a star on their high school football team. Should they be notable in the context of Wikipedia? I dunno. I've read some good debates on the topic and all I know is that it's really hard to think of universal rules.
