Historically, publishers take on the cost of marketing and printing. Printing copies of a book demonstrates their capital investment in the work and that helps unlock distributors, retailers. Often publishers would supply editing and formatting/publication services too.

Now, for ebooks, the marginal cost of "printing" is close to zero. But editing and marketing are still expensive. I think there's a lot of momentum towards preserving the historical royalties even if the business has changed.

The music industry seems to have made this leap to electronic distribution (near exclusively in their case). If royalties have been upset in that industry then we should probably expect something similar for writers.