Passports are inherently decentralized, which is needed because not all countries cooperate with each other - or have the same budget for technology/security. It's really way something at global scale could work.
(There are national-level databases, but presumably not every country has access to every other country's database.)
I struggle to imagine international airport without a credit card reader. Maybe some borders in some countries could've struggled before cheap ubiquitous internet, but not anymore. And even then it's their problem.
Countries don't need access to database. They need to validate public key / hashsum is valid (or something along those lines).
(There are national-level databases, but presumably not every country has access to every other country's database.)