I mean I just don't see the difference between this and getting any PC and slapping SteamOS on it.

There's not currently a way to officially put SteamOS on Steam* hardware. Plenty of people have done it but there's the usual compatibility issues where the image is built for the very specific hardware Valve installs it on so there's often wake from sleep and fan control issues. All solvable but it's not the level of turn key of even a mainline linux distro.

probably the "slapping steamOS" part of that