As I read, when tryiing to understand this, was that the aluminum for high-quality billets is cold-extruded. This causes grain to be tight and aligned. I don't think you can cold extrude steel and cold rolling does not give the same affect as forging.
Anyway, I'm not a metalurgist. If we have metallurgists here, it would be great to have their thoughts on this. I think one of our old friends MG or Fido is a metallurgist or materials engineer, but we haven't heard from either of them in a long time.
Richard L.