Because it’s a shelf.  

Probably because it would significantly add cost and complexity to the product and not that useful to many people looking at buying the shelf.   

Currently, there are no electronics in the shelf at all, so you’re talking about adding a power cable, transformer block (or whatever their called), and wireless charging pad embedded in the shelf itself, with versions for every electrical standard across the world.  You likely would need to have an normal power outlet for the Sonos One too, as no one would want two power cables (charger and Sonos One) coming from the shelf.  Oh, and since iPhones now have the magnet charger, you’d have to have versions for that too.  Probably couldn’t make the shelf reversible anymore.  Since we have a world wide chip shortage now, it’s probably going to be a lot harder to keep it in stock now.

So I’d say, if there was a wireless charger built into the shelf, it would probably be at least twice as expensive and you’d be waiting 6 months to get one. 

Oh, and a lot of people wouldn’t want to pay the extra cost, especially if they want to buy a pair, and have no desire to charge a device that’s not in a convent place for them.  They just want to place some nice fake flowers next to the speaker.