Long story short: money, dear boy.

More in-depth: some people like owning a physical book. It's easier for them to carry around, and not everyone has, can afford, or wants a big smartphone or tablet. (I myself have the smallest smartphone available, explicitly for the purpose of playing Angry Birds, and have no desire to upgrade or get a tablet.) It's also a nice nod to the English fanbase, since such books are routinely released in Japan (see: Ultimanias) but are never translated and released elsewhere, leaving a lot of details out of our hands unless someone is willing and able to buy and translate one.

It's a bit of extra cash in their pocket, sure, and it can't be updated, but some people are willing to pay for it, so they'll release it.