Maybe cost too. Custom 7nm silicone aint cheap and the steamdeck is quite an affordable product considering Vlave don't have access to the same economies of scale that Sony, Nintendo or Microsoft do.
Don't underestimate them, Valve is similar in revenue to Microsoft Gaming Division, so I think they can afford selling hardware at a loss.
Although I wonder how much exclusivity you get anyways with how remarkably similar the PS4/Xbox One and PS5/XSX designs are.