Physical passports, in the same way as physical currency, have numerous mechanisms for reducing the ability to forge the documents.

So these documents can be checked locally without any form of communications to some central authority (which doesn't exist across national boundaries).

They have visible anti forgery like UV printed symbols and information, underprinted background text and patterns, etc etc.

So they are more "meaningful" than an offline smartphone with a passport app in that they do not require anything other than the officer's ability to see, feel and read the documents.