Thats more of a perception problem than an actual fact. It turns out that hosting/supporting ebooks does have incremental costs and people dramatically over estimate how much the price of a hardback is the physical copy. If they just priced the difference in physical production vs ebook support into the price, I think most people would be surprised at how little the discount would be.
Now there is a difference of course but the greater part of the price of a book comes from the creation of the intellectual property (including subsidizing all those books that don't get sold) and the sales process.
The bigger issue is that publishers have lots of expertise in selling physical books (100s of years) and ebooks are brand new. Further, the biggest platform for ebooks is run by a company that is not in anyway aligned with the publishers business interests and is in fact a competitor. This is not true of traditional book sales channels.