Wikipedia:Notability is important

Many new Wikipedians and outsiders question our notability guidelines. They say that space is not an issue, that notability is a nebulous and ill-defined concept, or that Wikipedia should be about everything. While it might be nice to have articles on every subject imaginable, it won't work.

Verifiability
Many non-notable subjects are inherently unverifiable. If a page doesn't meet the general rule, which essentially says that there should be substantial third-party coverage in multiple reliable sources, then how can we expect to thoroughly source an article about it? Even if good sources do exist, it will be difficult to find them.

It is particularly difficult to find independent sources about a non-notable subject. The creator's website is not enough, for example, to discuss the impact of a product on popular culture, since the website is too close to the subject to be a credible source (and it is also likely self-published). While such sources are allowed, they are insufficient on their own, and are often biased.

Neutrality
People usually write on subject areas they are familiar with, in Wikipedia and elsewhere. If a subject is non-notable, relatively few people will be familiar with it. Of those that are, many will have positive feelings about it. This type of problem happens even in large, clearly notable subject areas. While we do have ways of dealing with this, our resolutions tend to upset and drive away the users most familiar with the subject. This leaves the article with little activity.

Inactivity
Important articles such as Barack Obama tend to have a lot of people watching them, which means that vandalism tends to be reverted quickly.

Not so for the non-notable article. It is vandalized, and if the bots and edit filter miss it, that vandalism may stay put for months or even years. Now, this can also happen to notable articles, as it did in that case, and now our policy on biographies tends to limit damage. But there are, as of September 2020, over 6 million articles on Wikipedia. There are only 130,000 or so active users. Now, we can quibble about whether that's an overcount or an undercount, but that's still an order of magnitude less than the number of articles. We can't keep up with every article. And, of course, vandalism isn't the only issue. Articles become outdated as new information comes out. Articles may need restructuring or cleanup. Right now, we do a shockingly good job at these things, considering the numerical disparity, but we still have about 2 million stubs, which require expansion. Throwing non-notable articles into that mess certainly won't help anything.

Audience
The audience for the article on Barack Obama is mostly everyone, or at least everyone in the US, regardless of how much or how little they know about the man. The typical audience for a non-notable subject consists primarily of fans of that subject, who already know the subject quite well. So there's little to gain from having such articles, from an encyclopedic viewpoint, since most people aren't going to look it up anyway, and those that do won't gain much from reading it.

Conclusion
Articles on non-notable subjects are difficult to verify. They are often biased, and can be influenced by biased sources; moreover, their authors tend to become upset when this bias is corrected, resulting in much drama and, ultimately, a lack of contributors for the article. Without enough contributors, the article languishes, gathering dust and vandalism. And the only people who will look at it already know it well enough anyway! There is no good reason to have articles about non-notable subjects.