i dont think its that its just to spur more competition imo. Handheld gaming PCs have been a thing since like 2015, but it was almost exclusively chinese conoanies only producing it over a kickstarter and some grew big enough to have a amazon presence (GPDWin). the lack of competition led to high handheld price costs that are reletively speaking, terrible (over 1000$ for what is essentially a handheld netbook).

the steam decks price was to garner interest and grow a market, which it will likely want other conpanies to take the reigns to. Valves decisions has ALWAYS been about growing a new market (steam machines(linux console pcs), steam link(porting pc gaming to the TV), VR(self explainatory), Steam deck(on the go linux gaming))