I think the problem is about where does work happen.

Sitting at a desk is the symbol of getting work done.

Tools like pen, paper and desktop computers have evolved around the desk + chair model. But I can't sit at a desk all the time. I'd rather move freely and choose my spot and body configuration depending on my mood.

I do desks because that's where I find the tools I need. I need Cinema 4D, Photoshop, After Effects, Sublime Text, note editors, web browsers, file compressors, server files uploaders, as much processing power as possible, and the ability to handle and switch between tens, sometimes hundreds of elements to ship a project.

Ideally I'd like to do all that wherever I want: sitting, standing, walking, next to my friend, inside, outside, going to bed, waking up, in the kitchen, in the toilets, all with maximum compute power and energy autonomy.

That's why mobile spread out wider than desktop. It's about the mobility.
Below is one of the series of tweets that prompted this post:

"No need to debate "kids use phones" or "iPad sits in a drawer". Reality, laptops sit on desks, used less by most. Mobile = important."

Steven Sinofsky

Right now mobile devices, i.e. phones and tablets, have nowhere near the power and speed of use PCs provide. Current mobile devices are extremely frustrating for people having complex workflows: 3D graphics, software development, video editing, etc.

The future is keeping the mobility of mobile devices:

While matching the speed and depth current desktop computers provide, so you could:

Creative people dream of continuous tooling availability, but the current crop of mobile hardware and software isn't capable enough for digital creative workflows.

Update 7 April 2017

The question isn't whether or not we can create on mobile—of course we can. Creation has been mobile for thousands of years. Creative people can create with anything. The question is: how will we do on mobile what we currently do on desktop?

Further reading