Why isn't the Speaker a retired judge or current judge who is appointed by the courts?

This is a really interesting question Peter. 

The Speaker of the House of Representatives cannot be a retired or current judge appointed by the courts because it would be against the principle of the separation of powers.

The Australian Constitution says that the power to govern the country is divided between 3 groups: 

• the Parliament - makes and changes the law
• the Executive - puts law into action
• the Judiciary - courts and judges who explain what the law means and decide if it has been broken.

Each group has its own job and must stay independent of the other groups. This help make sure no one group has too much power.

It is also a rule in the Constitution that the Speaker must be a member of the House of Representatives. This means they are part of the Parliament - not appointed by the courts. Also, judges belong to the Judiciary. If a judge became Speaker, they would have power in both the courts and Parliament, which breaks the rule that keeps these roles separate.