Probably because of the fuselage fatigue has rendered them unsafe to fly.
Alternatively, F-5 has an equally good characteristics of a trainer and Iran still operates a large number of them. IRIAF can convert F-5Es to F-5Fs to make the most of it's resources once the F-5Es lost their combat value. Just as it was done to Iran's F-5A's that were converted to F-5Bs to extend their service life until the end of their fuselage's operational life.
Probably. However the fuselage can be put into 0 hr flying condition (the fuselage is mainly made of aluminum). Boeing still operates two of them and many have been used as test platforms over the past. They could involve students, universities and flying schools in the program. As a matter of fact airline pilots also need training. Besides there's plenty of Rolls-Royce Nene Klimov RD-45).
Canadair proposed an upgraded version of the T-33 which operated till a few years ago.
Here’s an upgraded Bolivian T-33 project.
PS: The T-33 is still "paraded" in Iran!!!