No real reason. So what if it's fresh brass or used brass? It's not the matter of if it's shiny and new, it's the matter of what it does in the end. 

This way we know that the spent brass case he's working on at the moment has performed it's job in the past. Blow out the neck add a few hundred more grains, and you're just as likely to have a working wildcat. It doesn't matter in the long run. The one round he was working on was an old case. The next round could have been new brass. Who cares where it came from, what matters is the end result.

That being a wildcat 5.56 or 7.62 bullet. With maybe 80 to 120 extra grains of propellant, which really doesn't sound like much, but can make all the difference in the world. What goes from a 2 inch hole in a wall, will become a 6 inch hole in that wall, and the the corrugated steel behind it, and punches through the two guys behind that steel as well.