It's not only that, hardware-only companies have been building gaming laptops and PC's for decades. It's also a question of economies of scale. The more devices sold, the cheaper it is to produce each one of them. Valve has a much stronger position and brand from which to produce and market such a device.
In order to create price competitive gaming console companies need to control something else in the value chain such as software sells. Even though Valve didn't completely locked down Steam Deck like other console makers it would still put them in very strong position to extract enormous software revenue if Steam Deck achieve wide adaptation. This is why Valve can afford to release better and more polished hardware at only 1/2 ~ 1/3 MSRP compare to other similar handheld PCs currently on the market.
Other OEM simply have no incentive and maybe can't afford to lower the price if they can't make up for it elsewhere.
Steam OS is also an important piece to the ecosystem. Win10 has too much bloat, not doing well on battery life and breaks stuff constantly. A free & reliable OS customized for handheld gaming on x86 is pretty much required to challenge console/mobile gaming.
This is all pretty obvious, but the points made are good.
I'm wondering if any other companies are planning to use Van Gogh, which could be a great 7nm budget chip in the long-term. We already know of the Dragon Crest refresh on the roadmaps.
If Steam Deck is successful for Valve, what would a Steam Deck 2 or 3 look like in a few years? The 5nm, 3nm, and 2nm nodes are going to be crazy for this kind of device. For example, 8 cores, 1080p target resolution, and better battery life by 3nm.
Microsoft won't make a handheld XBox so Valve have done it themselves. The Steam Deck has the potential to fire up a fledging category of handheld PCs for gamers and not just enthusiasts.