The classic example: for every loaf of bread a baker bakes, the variable costs (raw ingredients, fuel, labour) are approximately the same for each extra loaf baked, so his margin and profit per loaf don't magically increase by baking more.
Contrast this with someone creating a copyrighted work and receiving royalties per instance sold or performed. Here, increased volume does increase margin and profit per instance, because the real cost of labour the creator must put in to receive royalties on every new sale or performance tends to zero. (I'm discounting the cost of marketing etc., which in most cases has only a second-order effect and doesn't change the final result.) Add to this the income which copyright brokers receive from media distributors etc. without having any part in the creative process, and things become very unbalanced real quick.
The root cause of the problem is that it is extremely hard to put a realistic monetary value on the act of creating a copyrighted work, as it would have to take into account the real effort required to create it, as well as the total (mainly intangible) benefit to all who will ever get exposed to it.
It would be much better if the artist could make dealings directly with the audience, just like the baker does with his customers.
As Syd Barrett put it: "All middle men are bad."