Two reasons. We want this product to be used regardless of battery power in devices or access to computers. Also, there isn't much benefit in learning to read by screen versus the standard way in print. Since children are inundated daily by images on screens we found greater benefit in them being able to shuffle up the cards, categorize by color, by era, or by their favorites. These are things an app just can't provide. Plus, you don't OWN your app or ebook. Once that company goes away, so does your data.