For example, Dragon Age: Origins is a recent game, and is a 20 gigabyte download. Downloading The Gathering Storm (a particularly large book), is around 2 MEGAbytes. The game is. . .what, four orders of magnitude larger than the book? Most of the US just isn't set up for that kind of download yet. After all, the average broadband speed is 3.9mbps = 500KBps, which means an 11 hour Dragon Age download if we assume perfect network conditions. Given that sort of time, consumers will undoubtedly run into issues of ISP throttling, resuming downloads, etc.

That's as opposed to a book, which anyone (even dial-up dinosaurs) can download in a reasonable amount of time, with a minimum amount of hassle. To distribute games in a primarily digital manner we have to solve matters of economics AND infrastructure, whereas books already have one of those taken care of.