There is literally too much content and most of it looks awful.
I took a look Amazon's kindle unlimited this afternoon and what I saw were an incredible number of science fiction authors that I never heard of, pushing out what the blurbs and titles made look like bad romance novels in space.
The functions of the editor and publisher are just missing from this mish mash. If you look at paper publishing it's a large financial commitment to publish and market any given book and most would never pay back the investment. Hence publishers to market the works and editors to select quality material were immensely valuable and helped make certain that if an authors work was published it had a better than random submission chance of earning back it's costs.
Now the cost to "Publsish" as an e-book is minimal and much of what would never have been published in the past is flooding all over the place. So you have lots of "Authors" self publishing and not making money. This really shouldn't come as a shocker. The problem is there are so many of them they overwhelm everything else. If I read correctly Kindle Unlimited has 600,000 titles. It's just numbers but there really just aren't enough people in the world to see that most of those authors make a living from being published there.