(I realize that this is more complicated than "70%", see https://kdp.amazon.com/self-publishing/help?topicId=A29FL26O...)
I think it's a sign of efficiency that a large portion of the money goes to content creators. Content creation will get cheaper as technology improves, sure, but in fields like music and writing, content creators struggle to make a decent wage. Therefore as content distribution technology improves, the percentage of money given to content creators should increase.