[chiefr]

Having been doing this reloading thing for now over 40 years, I can remember quite a bit of new brass from the factories was pre-primmed. New unprimed brass was not always available and difficult to find.
These days, I rarely see pre-primmed brass and frankly people of my generation did not like it.
Fact is Murphy is always around. Some of the pre-primmed stuff would get wet, or tarnished if it sat around to long despite how well you protected it and people wanted to tumble it.
Also, and even worse, case length/shapes varied in new brass considerably and folks prepped their brass accordingly. Comp shooters who reload want particular primers from a different mfg or same lot, thus a myriad of reasons pre-primed cases dissapeared from main stream.