I guess most of it is because these light aircraft just won't die. These are simple, well built machines, and treated well and with the appropriate maintenance, they can last essentially forever. Because they have been in use for so long, we know just about every failure mode, and they were build in a time where the idea of safety was comfortable margins and aerodynamic stability. It results in robust machines suitable for training.
Sure, they are slow and inefficient, but these are not airliners where the point is just to get passengers to their destination as effectively as possible. Here, flying is the entire point, who cares if it is slow? And the fuel costs are less than buying and maintaining a more modern airplane when you already have a 50 year old but still usable 172.
Clean sheet airframes exist of course, like carbon fiber, high performance machines, good for those who want high performance, but usually, that's not a priority for flight schools.
Sure, they are slow and inefficient, but these are not airliners where the point is just to get passengers to their destination as effectively as possible. Here, flying is the entire point, who cares if it is slow? And the fuel costs are less than buying and maintaining a more modern airplane when you already have a 50 year old but still usable 172.
Clean sheet airframes exist of course, like carbon fiber, high performance machines, good for those who want high performance, but usually, that's not a priority for flight schools.