Because it's not always pages and a modern smartphone camera can easily compete with a consumer-grade flatbed-scanner. I actually do own one of those, with a feeder (though not duplex) and it's a hassle compared to using Scannable.
My current setup - excuse the mess on my desk, please - just required an additional $8 adapter for an run-of-the-mill tripod, but other smartphone holders, like a gooseneck clamp work too.
Put Scannable on Default capture and start putting your items on the desk - papers, letters, book covers, etc - and Scannable will snap them reliably. But it's unforgiving when there is a small distraction - your attention comes back and you find dozens of identical pictures in your session.