Well once you have a CNC mill the only difference between machining the full LR and finishing a blank is in run time.
As to why, I obviously don't know. But the LRs being the only regulated, and at the same time trivially machinable parts probably have to do something with it.
Per the article and what I understand, there's moderately skilled work required in sourcing and perhaps adjusting the right CAM data for a complete billet -> lower receiver machining job. Plus a lot more machining involved; as I understand it, a drill press can finish an 80% receiver. But I suppose such skilled people can be persuaded to do this sort of work for a cartel.
The difference between programming the 20% finishing and machining the whole part is not that large. Moreover, the finishing would require manual set up and zeroing out each semi-finished piece.
Aluminium is cheap material and machines really well, there's not much downside to machining the whole piece. Plus, you save the expense of buying hundreds of pre-fab LR blanks.
As to why, I obviously don't know. But the LRs being the only regulated, and at the same time trivially machinable parts probably have to do something with it.