Salesforce says the following:

"Customer Community, Customer Community Plus and Lightning External Apps licenses require unique usernames whithin the Salesforce Org. that a community belongs to."

"Partner Community licenses, Employee Community, Lightning External Apps Plus licenses require unique usernames across all Salesforce Orgs that the user belongs to."

Instead of having to memorize that, I would actually like to understand why (the underlying reason(s) for the uniqueness of the usernames being within the Org OR across all Orgs globally to which the user pertains.

Is there any logical reason that you can think of?