Ask Lee Child and he’ll happily tell you his most famous creation comes from a long line of vigilante outsiders.
He wrote based on instinct - which is to say, based on everything he had ever read and subliminally absorbed before. And sure enough, Jack Reacher turned out to be a man of rank (a major, a West Point graduate) now cast out from mainstream society (albeit voluntarily, by his own hand) and dispensing rough justice outside the mainstream rules.
As such, Reacher slotted very neatly into the folk hero tradition. Specifically the knight-errant subgroup, which seems to be universal in appeal. Like in the Japanese ronin myths where a samurai, disowned by his master, is sentenced to wander the land, doing good.
For Lee, the trope was so strong it overrode common sense and quotidian realism. Think Batman and a slew of Marvel characters
Everyone feels they’re misunderstood. Everyone feels like they’re an outsider.
Hence the universal appeal.