It doesn't have to be the superior way to create everything. Rockets require virtually all custom parts. Custom parts require custom machining and don't get the benefits of economies of scale. Having one machine that can create all of those custom parts cuts out the requirement for custom machining and they don't need to change the machine if they make adjustments to the parts. Strongbacks on the other hand are relatively simple truss structures. You can use mass produced parts to build up the strongbacks fairly easily and inexpensively. There's really no reason why they couldn't print the strongbacks, but it doesn't really make sense to.