Except that they have a lower level API and as they are a single hardware target games can be heavily optimized, Steam Boxes do not have these advantages thus need more powerful hardware for the same result. Nor do they have a manufacturer willing to have little profit or a loss on the hardware in order to get them in the hands of consumers.
For some reason people expected both games optimized for Steam Boxes as well as cheap hardware, neither of which were ever going to be a reality.
I'm not, but it sounds like you are. Firstly, 3k is not 2048x1536, it would be 2880x1620.
A steam box is about gaming, why would anyone buy one to drive 4k video? Gaming at 4k is not trivial even for today's high-end GPUs, go try it with your i3 and $100 video card.
is only what, $50 compared to $350 for an Xbox One? If you're looking for PC -> TV streaming that will be far more economical than buying an Xbox.
Steam Boxes have always had questionable value, they aren't going to be as efficient as consoles and do you really want that gaming PC in your living room?