If this is the ideal model, why don't authors skip the middleman themselves and just put the .pdf/.epub on their website directly in exchange for donations?
On the same note, why do game devs need to give Steam 30% of their money and not just sell to the public directly and pocket the 30%?
Maybe because those middlemen platforms provide a combination of discoverability, user review rating system, network effect, convenience, and trusted return policy at scale that's valuable to both consumers and developers/authors enough for both parties to tolerate it as the status quo even if it's not perfect, it's just good enough to be the default.