Agreed. Its a walled garden.  Easier to maintain since reading is time expensive but just like music, it’s becoming more decentralized for people to self publish.
I recently met a woman that was a writer and I could only buy her book on kindle. I surprised her by reading it to surprise her but the real surprise was how amazing unheard this book was. 

 That's cool 💚
I quit buying books through amazon a while ago. I'm thrilled when an author has a direct-buy option. 
 

 100% -they should get more than a single digit percentage of the proceeds