Prior to 2009, when publishers scoffed at the ebook market, they offered writers contracts which gave us half of the money they made off ebook sales.
Authors can easily produce ebook versions of novels and shorter work which publishers don't own.
Authors will make far more on those ebooks through direct sales than publishers are offering. There is no incentive for authors to sell those rights to traditional publishers which means, in the fairly short term, publishers run out of material to sell.