Also a fraudulent covid vax certificate is of more value than a fraudulent yellow fever vax certificate. In Australia, for the majority it will allow you to get out of your pyjamas and past the front door.You make it sound so easy. It is the high tech solution that is the problem. It's not difficult to come up with a vaccination card to be used worldwide like we have all used for yellow fever forever and a day. But when it comes to the high tech "solution", every country has a different protocol, a different app, or no app, or no centralised health system, even approval for different vaccines. Finding a one size fits all solution like a piece of card is going to be impossible which means pretty much every country negotiating with every other country, unless the WHO get involved. And whether they do or not, that is going to take forever.
And then you have politics (and competence). At the moment, you can enter the UK without quarantine from an amber country if you have Tony Hancock's US pharmac_ stamped card (probably with or without having had the vaccinations). Yet as a UK citizen, I would have to quarantine for 10 days despite my two AZ vaccinations - and solid proof. The UK aren't nearly as interested in their overseas citizens as they are in kowtowing to the Americans. Sort of understandable as the number of entrants from Oz will be small for obvious reasons, but the same applies to all the UK expats in the Middle East, Singapore, Hong Kong, etc.
Also the risk of yellow fever, given so few travel to/from the affected countries, is tiny compared to the risk of covid, especially if new variants come to light. So we do need something more robust than a piece of paper even if it will be painful getting there.