There are also some productive things that are doable with mobile devices. For me that is learning flash cards with Anki and data acquisition for openstreetmap (house numbers, etc.).

The problems they have is usually the small screen that makes it hard to view much content at once and that text input is really burdensome. Some tasks require these and are thus not suitable for the mobile devices. They however also have advantages to be usable in many situations computers aren't (for using a computer while standing you need a special desk while you can use the mobile devices in almost every position and place) and touch input may be better for some tasks.

For stuff that would be possible both for computers and for mobile devices equally good there's the barrier that it's already been developed for computers and that everybody needing it for work already uses a computer, so you would need to put a unbearable mass of work into it to bring it to already is with computers today.