Most identifiers are for use within a system, and are not intended to be GUIDs, semantically. My name is Paul Davis, which functions well enough as an identifier within my lived community, but is pretty useless as "real" identifier for me, which is why other entities I interact with want my birthday, or social security number, or passport or ...

One can cheat on this for airline systems by taking the position that "the system" is the aggregation of all the different systems, not any one of them.