There is, of course. The book is interesting within a particular range of cultures, and depends on good translations to be popular outside its native language area.
So, for a hyper-successful book, like "Harry Potter", the potentially interested audience is "merely" a few billion people, and "merely" a few hundred million are potential buyers of the book. 600M books sold worldwide, counting all the books in the series.
Selling 2x as much is highly unlikely, and 10x as much, completely unrealistic. The number of New Testament copies printed since 1450s is estimated to be below 7 billion.
This is different from in-person performances, where again giving more concerts is more effort, though giving the concert in a bigger stadium for 5x as many people is again not more effort.
The point is the scalability, or in other words low marginal cost of sales. It becomes a commodity.
Most regular jobs that people do also don't scale. Even in software, custom one-off software for a particular company is still the rule, not the shrinkwrap or public SaaS.
In a more profane context, it's like the difference between a prostitute and an OnlyFans performer. The prostitute with the most clients won't have 100x as many as the average, while orders of magnitude difference in subscribers is common on OnlyFans.
Media takes time to consume, which means that many products won’t be consumed at all even if they are good, much less be consumed enough to reach the point where word of mouth can achieve virality.
The reality is that the best winner is usually a product good enough to not be laughed at (though not usually best in class), with some characteristics that make it marketable, and with strong backing behind (money, connections or both).
One funny effect is that you end up with artists talking about viral topics like social justice and discrimination like it’s first hand experience, but a quick check will show they come from immensely privileged backgrounds.
I assume the parent comment's last paragraph is why every fantasy novel I've read in the last decade has a five-page-long lesbian romance; the author has to play ball with the publisher in order to get marketed.
I think all disciplines start “hard” and objective, getting into the subjective at a more advanced stage - at the very least it is easy for a knowledgeable reader to know when an author is actively choosing to disregard an established rule and when it is incapable of following it.
Saramago, the novelist, is famous for writing unconventionally, with long phrases and sparse punctuation, but nobody doubts he could have written conventionally. Same for Picasso rejecting realism.
Many famous authors right now would clearly have a hard time following the “base” rules, and that for me warrants judging them by hard rules. If they could, and got into the twisting rules phase, then I agree it becomes a far more subjective matter.
Books, music, films, games, basically all creative things. Because everyone can just watch/read/listen to the best of the best. It's not like restaurants where the consumer actually consumes the product.
It's the result of global scale distribution networks. Long ago you could be okay-famous by being the best guitarist in the village and people would appreciate you. Or you could be the local history buff. But if you write enter the global arena, you'll compete with everyone else. There's no limit to how many books the famous author can sell, or how many times a famous singer's song can be streamed on Spotify. There's simply very little demand for mediocre stuff. Also, people want to be able to talk to each other about cultural things. Even if it's a bit more fragmented today, people still want to belong together through liking the same things.