The reason Qantas has a cadet scheme is simply because they make money from it. You take 10 guys/gals who have each paid $120K and you take a handful on as S/O's at the end of the course.
Think about it. You make 1.2 mil from hiring 10 cadets ! How much would you make if you hired 10 pilots from GA/military/regionals ?
Answer: Squat.
Remember airlines are no different to any other big business......they are run by accountants.
In many of the big carriers hours and experience holdd a lot less weight than they did many years ago. The bottom line is save money and make money.
My theory anyhow.