But you can share with others. You can even sell the book.

It is very convenient to have hundreds or even thousands of books in a tiny device, especially for someone who has very little space to keep physical books (and those who move a lot). While nothing can replace physical books, ebook readers are a fairly good compromise. Plus, it is possible to reduce the cost of books (and less dead trees). But we can't share, unless we share the ebook reader device itself :(

As with most things, humans will put commerce first, and I don't see this situation changing anytime soon. Amazon has won the ebook reader race and they are likely going to stay there for a while.