In many cases, for book contracts more than a few years old, the publisher does not have the ebook rights. Authors and agents aren't necessarily going to jump to sign new contracts at a time when standard ebook royalties from publishers pretty much stink and the whole business is in a state of flux. On the other hand, there's a growing number of publishers like E-reads that specialize in putting backlist into ebook (usually with better royalties). And of course, there's a rapidly growing number of authors who decide to do it themselves: considerably more upfront work, but definitely better payoff.
As one of the latter group (at least for some of my backlist), I'm part of several groups of authors giving each other support in getting their backlist books out as indie publications. (
Backlist eBooks being a chief example.) One thing that's surprised me is the way romance writers have left science fiction writers in the dust in getting into this business. Sure, there are some terrific SF/F writers getting their books out, but as a group, they've been remarkably slow to jump on this train.