Android = Steam. Nexus = SteamBox.
Android may equal Steam but Nexus does not equal SteamBox. Google commissions Nexus phones to give consumers access to an non manufacturer and carrier modified Android phone and a streamlined and fast access to OS upgrade. When you use Steam you don't get an UI designed by Dell, HP or any numerous PC vendor nor when you open Steam do you have to worry about your library containing undeleteable apps from Best Buy or New Egg.
When Valve releases a new version of Steam you as a PC owner don't have to wait for approval by your PC manufacturer and your PC retailer of choice to approve and make it available to you (which can take months). You get the Steam that Valve intended, you don't with Android phones hence the need for Nexus. Furthermore, while Nexus is critically well received, it is produced in rather low volume and availability is always an issue.
For Valve to enter the software market, they only had to developer Steam. The risk of hardware to run it was placed on Dell, HP, ASUS, and IHVs.
Thats an apples to orange comparsion because Dell, HP, ASUS and other PC IHVs hardware sales aren't dependent on consumers' acceptance of Steam. It helps but if a consumer is dead set against using Steam, deleting Steam is an option and the hardware can be judged on its own merits. If a consumer is dead set against Android then Android based phones won't be considered for purchase.
After Google's OS, they then decided to create their own hardware range, which they didn't have to do. Valve are considering the same. Whatever issues Valve would face, they have already been faced by Google with Nexus hardware from IHVs. That doesn't prove the venture will be successful or not, but it does prove a software company can commission hardware and sell it. Design, manufacturing, distribution, needn't be crippling (unless Google have lost gazillions on Nexus, I've no idea!).
Again Nexus was created to give consumers access to an unadulterated Android phone as Google intended. Manufacturer are happy to manufacture the Nexus because it gives them a chance showcase their hardware and build their reputation with serious Android users.
There is plenty of proof that a software company can commission hardware and sell it. But the motivation of Google with Nexus doesn't serve as a good model for the Steam Box. Nexus is more of a marketing device for the OS developer and hardware manufacturer and probably not much of a profit center for Google. Google has done a very poor job in venturing into hardware and will more readily serves as a good example as what Valve shouldn't do.