I think the situation is somewhat more complicated.
Generally, the safety factors are less in aircraft than automobiles--far less. When you build things that way, you put the materials further up the stress-strain curve and cause fatigue failures far sooner than they would occur if there were more of a safety factor.
I believe that the changes in the material that characterize fatigue are invisible. Fatigue tends to lead to a crack and crack propagation (which hopefully is caught at inspection), but that is the best case.
The crack is visible, but the material changes that preceded the crack are invisible. There is no visible "pre-fatigue" or "pre-crack" even though changes in the material have already occurred.
I suspect that some engineering designs may fatigue or alter materials in a way that tends to lead to catastrophic failure rather than something that shows up at inspections, such as a crack. Or, perhaps the first crack and the failure may be very close together in time--perhaps far shorter than inspection intervals.
I wouldn't know what characterizes such a design, but I would suspect that such designs exist and that the design features that cause this are subtle.
This is just a suspicion.
I think it may be far more complicated and far more subtle than the accumulated effect of hard landings.
When you have a structure that flexes and vibrates, I think odd things may happen over time.
This post has probably caused famous deceased materials scientists to roll over in their graves. I'm just guessing ...
DTA