Pirke
This is a very complicated subject and one that can only be descussed on an aircraft by aircraft basis.
For example the Bulldog is limited by the danger of fatigue cracks radiating from bolt holes in the main spar, it would be imposable to find these cracks on an annual check without removing the wings and doing NDT checks. The cost of such "on condition" maintenance is prohibitive.
On aircraft such as the PA38 I suspect that the fatigue life is more a function of where the manufacture stopped testing the airframe with a view to not wanting to support an airframe past that age.