The world isn't limited to books. A lot of companies own the rights to large libraries of film, video, audio, and short form text. Building original works with existing stuff is a lot easier than shooting, recording, and writing everything from scratch, and you have a lot more freedom to mess around and adapt everything to work in an interactive context.
You still need developer talent and you can't just recruit anybody. Building entertainment software requires a different skill- and mindset from that of building an e-commerce website. People that do entertainment-oriented interactive media are few and far between and usually get to work in environments that are considerably less frustrating than today's e-reader platforms.
Which is why you end up seeing a lot of native apps or websites and very few interactive e-books proper. The talent goes where the tools are.
The alternative is to train your own people. Then you let your staff pad their CVs with a couple of your own titles before they head off into other fields. Fields that give their developers actual tools to accomplish their goals and documentation on how to get there.