Because YOU do not load the data since anyone could just say they were vaccinated or show a fake card. The place of immunization would have to report the immunizations to a third party and right now that is not allowed in FL because of the vaccine passport ban.

There are apps that kinda do the same (QR code generation) but it remains to be seen if the State of FL will eventually allow the data to be release since this goes against the no vaccine passport mandate

Many were vaccinated in state run facilities and those have even less of a chance

Note: I am only commenting on FL because that is where I received the shots