> How do you look for a person, if not based on his/her SSN?
These are different concepts. "Looking for a person" means search. You can look for people lots of ways. In medicine for example, they often look for first name + last name + birthday. Is that a unique ID? No, but it's close enough for search usually.
On the other hand, for any kind of indexing or foreign keys, you want an actually unique, immutable ID, which means you don't want a natural key, you want an artificial ID created just for your database.
These are different concepts. "Looking for a person" means search. You can look for people lots of ways. In medicine for example, they often look for first name + last name + birthday. Is that a unique ID? No, but it's close enough for search usually.
On the other hand, for any kind of indexing or foreign keys, you want an actually unique, immutable ID, which means you don't want a natural key, you want an artificial ID created just for your database.