Arduino wasn't strictly confined to Atmel/Microchip for a while now. Their newest mainstream boards roll with Renesas chips.

And Qualcomm itself is not in the business of making mass market MCUs. Does Qualcomm want to be?

They can, they already have the kind of dies they could put into those. But they would be competing against the likes of ST, and they wouldn't have the wide ass margins they're used to.

They would also have to be writing public documentation, and dealing with hobbyists and small developers. And the impression I got from dealing with Qualcomm? They'd rather douse themselves in gasoline and set themselves on fire than acknowledge that small developers exist.