I learned about Sturgeon's Law (90% of everything is crap) during these events. Most people think what they've written is interesting, or unique, or worth publishing. However, publishers have to make money, so they curate very carefully, edit the hell out of the raw manuscripts, and then only actually commit to publishing what they think there's a market for. Nearly zero of the random manuscripts that are sent in by unknown authors ever make it into this funnel, and the honest reason why, at least from what I have personally read, was because most of it was completely terrible -- in as many ways as you can think of terrible to be.
Three other thoughts:
1. The great videogame crash of '83 happened because the dominant platforms at the time did not have a lockdown on curation, so anybody and everybody flooded the market with garbage. Consumers decided it was better to not buy anything than to chance spending the equivalent of nearly $200 in 2024 money and get trash.
2. A few breakout, self-published, authors are making it through the piles of junk, but they are few and far between. Andy Weir (the Martian) and Hugh Howey (Silo) come to mind. The "secondary" film and TV markets are so starved for good new ideas that their works are getting converted into non-print media almost as fast as they can get sets built and costumes sewed.
3. There's probably a new market for solid, well known, curators. People who make money sifting the dreck to find the Weirs and Howeys and surfacing the cream to the top. The creators are self-publishing, and the publishing houses aren't doing well these days. But the curation function is still wildly important and finding the right way to do it, and the business model around it, is perhaps the future.