I think it's a combination of availability and experience, and motive. Let's take each in turn.
It seems (I've never done it) that it's fairly easy to copy music or movies into mp4 or mp3 format and strip the DRM. How would you do that with an e-book? What format would it be coming from? What would you copy it to? Is kindle the standard?
The lack of availability, means that you're more likely to find the book you want in the digital book store, and not have to hunt around for it on a torrent site or something. You also don't have to worry about the format of the book working well with the format of your device. Though this may have been an issue for music and movies at one time, I don't think it is anymore.
Lastly, what is the motive of a person who is going to copy or steal a book. First off, I've got friends in digital publishing at a major publishing house, and I've never heard of a time they are not concerned about keeping their jobs. They get paid nothing, and the idea that 'Publishers are having unparalleled profitablilty' is questionable. They have lower revenues per book, and lower costs per sale, but overall, they haven't been able to cut the overhead of editors, marketing, etc. to make the businesses more profitable. I don't think we look at stealing a book and think "the publisher and the author are making squilions of dollars, what's the difference if I get a free copy". It also appears that book publishers don't play the games that other media publishers play with respect to having the correct viewer rights in this region, delayed release dates, etc. etc.
We also spend much more time with a book than a movie, and though music may stick with us for a longer period of time, we are probably less inclined to consider an 'amortization' of the number of times a song is played version the amount of time we spend with a book.
If I'm going to dedicate time over the next 3 weeks to reading a book, I'm more willing to pay $10 for it, and I can always go back to it, vs paying $7 to rent a movie which is only available to me for the next few hours.