As an adult with kids, why would I want to spend my scarce time and energy building my own machine, installing and configuring shit when I can just buy this that is guaranteed to work well. Yeah, when I was 18 I'd probably do it myself, but I just don't have the patience for bullshit anymore.
I built my last computer with my kids, it was done in 2 hours and it was a family activity.
You'll also need the exact same configuration on this than on any other computer on which you install SteamOS.
There's also absolutely no guarantee that this will "work well". I fully trust Valve to be fair, but you're talking about a completely untested piece of hardware.
Grasping at straws.
I'm sure you don't need to build your machine just to install SteamOS on