Isn't the point of Valve's console that they want to get SteamOS out to people to test, not because they actually want to sell it to people? It looks to me like they're shipping with a variety of system configurations (one of them even has an i3 in it, certainly not a hardcore processor) and they're seeing how each of them holds up. They're going to leave it to other companies to actually make some real consoles (they might release their own too, but they're not the only ones who can), and one of those companies certainly could perform the system integration that you'd like to lower the cost.

The point of SteamOS is that it's open, and anyone can make them (and people other than Valve can sell them). If they become insanely popular, I'm sure that someone will jump on integration to sell a cheaper console.