My guess is that countries are still testing vaccinated people because travellers coming from places with all of: a simple paper certificate, anti-vax doctors faking them, and anti-vaxers themselves faking them, they simply don't trust them enough.

At some point though they'll have to give up, and admit some fakers (like those, but much less, who arrive on fake identity documents) get through the system. Anti-theft systems in shops don't stop all shoplifters, but they do stop and deter a good majority. That's life. Nothing is 100%