There are too many people who can't resist the temptation to get something for free if they figure they won't get in trouble for it. You see a small example every day in the produce section of the grocery store, where half the shoppers pop grapes or cherries or strawberries in their mouth, realizing full well that it's stealing. And don't forget that many people who are into computers enough to go to the bother of downloading an entire book are also probably Napsterites, who think nothing of downloading albums and songs of music despite the fact that the publishers and artists get nothing. Never mind that in the case of many artists, that's not the case. And never mind that even if it WERE true for music publishers, it's still stealing.
Books are more portable, less hassle.
You can dog-ear them if you want. They smell good. Books are far more lovable, portable and bendable than a computer screen. And I say that as a computer enthusiast. It would be nice, after all, to believe that we live in a world where honesty is the prevailing virtue of the overwhelming majority.