Expensive work. How are they going to recoup the cost? Sony and Microsoft do this by selling tens of million bits of hardware then leveraging licensing. Steam has no licensing for SteamDeck, they rely only on the revenue cut of Steam - which Sony and Microsoft also have in addition to licensing and masses sales of juicy profitably accessories like controllers, headsets and other bobbins.
That is the good thing for Valve. There are very small HW development cost since they can take whatever AMD/Intel (and in the future nVidia) has available for their thermal profile. It will just be a PC in a box running a Linux distro.
The SW stack will be exactly the same as Steam Deck, so you don't have to do anything extra there either.
Then, if the boxes are a success you can get other manufacturers to design and manufacture them, so Valve can go back to be the most successful game store.