It really isn't that complicated. E-books have been around for decades, they are considerably older than the Web. If people wanted to read books in their browsers, e-books would have taken off much sooner than they did. People had ample opportunity to read e-books on their computers, and decided not to.
One of the objections to e-books was people saying that they didn't want to read on a screen, but many people drop this objection when they try an e-reader, and find that it isn't like reading on a screen.