I know you could fit multiple phone chips or probably like an M series laptop chip (in the hypothetical world Apple made them open), but the pi and the peripheral ports hit for me that is a real computer
+the fact the pi is so darn cheap, has the IO and can emulate the game on the system not at all designed specifically for the task while also running a whole Linux stack and being online. Things have advanced tremendously.
The deck is pretty awesome. I think my big upgrade for it will be some AR glasses like the XReal Air. The idea of a big screen to play on pretty much anywhere really appeals.
I just have to see if I can wait for the next Valve VR offering.