The dependency accumulates one reasonable decision at a time.
Nobody decides to build an organization around one irreplaceable person. It happens gradually: the person who set up a system stays the only one who understands it, the person who negotiated a vendor relationship becomes the only contact who can reach them, and the shortcut nobody documented is remembered by exactly one employee.
This is not a loyalty or personnel problem to manage by hoping nothing changes. It is a decision about how much undocumented dependency the organization is currently willing to carry-and whether that amount is a choice or an accident.