It's marketing; stemming from an innate belief in adverts being reality.
Back in 1999 I remember some bits of a conversation about a Motorola Razr 3.
"I can take pictures with it; I can browse the internet; I can do this, that and the other!"
"Can you make calls with it?"
Since then, remember the adverts for Surface laptops full of street dancing kids waving them about and not actually using them? People flying around (unaided) in operating system ads? Madonna/Rolling Stones? It's the same with phones. As other posters have mentioned, stuff like the Apprentice has encouraged silly poses as if their users are in some kind of awe of these devices.
Look at me, I am supremely important in my worship of my device, that is why I'm adopting such a strange pose. I've spent all this money on it! Look! I'm really dead dead important having a conversation about buying cheap sausages from Lidl. What is cash? Oy! Mr Mopedmugger… come back!