I like the steam deck and use it as a controller, but you could make the same argument about the switch. Regarding the switch, my answer would be that while it is a controller, it is - IMHO - a poorly designed and uncomfortable-to-use controller.
While you and I like the ergonomics of the deck, others may not. One can hardly blame those people for wanting something they feel more usable.
I like the steam deck and use it as a controller, but you could make the same argument about the switch. Regarding the switch, my answer would be that while it is a controller, it is - IMHO - a poorly designed and uncomfortable-to-use controller.
While you and I like the ergonomics of the deck, others may not. One can hardly blame those people for wanting something they feel more usable.