Why peak output current is greater than short circuit output current in 7805 voltage regulator?
Those are two very different things measured in different situations.

Short circuit current is what comes out from the regulator when its output is shorted to ground, so the output voltage is zero. So regulator protects itself by limiting current.

Peak output current is the maximum current what the regulator can drive into load when output voltage is regulated to the nominal 5V voltage. This requires that the input voltage and temperature are within some optimal range, or there will be less current.