I believe that the instinct to over-engineer is based in part on bad prior experiences with trying to separate concerns after it's 'too late'. Either your own personal experiences, or those of your mentors.
Lacking any better skills to identify and avoid those problems when they begin, they try to stop it from happening in the first place. Fences get erected everywhere in case they might be needed, and they frequently turn out to be in not quite the right spot or shape. The code becomes coupled to the bad interface instead of to other code, and the fixes are just as bad.
YAGNI in theory is about trying to develop those other skills, but gets twisted into an excuse for bad tech debt loads.
Lacking any better skills to identify and avoid those problems when they begin, they try to stop it from happening in the first place. Fences get erected everywhere in case they might be needed, and they frequently turn out to be in not quite the right spot or shape. The code becomes coupled to the bad interface instead of to other code, and the fixes are just as bad.
YAGNI in theory is about trying to develop those other skills, but gets twisted into an excuse for bad tech debt loads.