The short answer to this question is "because we haven't figured out how to make it less expensive than regular site built construction".
The longer answer: prefab, or modular, construction has made inroads in various construction niches. Parking garages are nearly all prefabricated concrete for instance. Light framed buildings (either wood or cold formed steel) frequently have their structure panelized off site. Hotels (especially in places where the cold weather means a short construction season) are sometimes built using prefab modules.
It's also super common to have prefabricated lower-level components - wood trusses, bar joists, hollowcore panels, etc.
But any higher degree of prefabrication tends to be more expensive than conventional, not less. They mostly get used when their perceived benefits outweigh their costs. (If you look at a panelizer like Entekra's benefits page, they don't list "cheaper" anywhere). Two major exceptions to this are parking garages and HUD homes, which (among other reasons) are heavily optimized for a particular form factor.
Regarding Bone Structure, it's an interesting system, and I love seeing people tackle this problem space. But I rate the chances of a custom fabricated steel structure ever being less expensive than conventional wood framing as "very near zero". This seems limited to the high end custom home market, and their marketing material seems to indicate that as well.