Some rationale for blocking international travel - preventing the spread of international variants, and preventing the destination country from being overloaded by incoming infections.
Most nations aren't getting the vaccines yet. Vaccination distribution and delivery require complex logistics. Nations that don't have a good public healthcare infrastructure need time to gear up. Vaccinations have just started and are being given in a phased manner. Blocking International Travel (or at least mandating a 14 day quarantine) helps keep variants in check.
Ironically, it's in countries where there is partial vaccination that we haven't the greatest risk - partial pressure against the virus will select it for stronger ability to be vaccine resistant. It's why it's so important to give everyone two doses of vaccine as quickly as possible and vaccinate the entire population, so you don't have the virus spreading, mutating, and potentially evolving to become vaccine resistant and then endemic in the population. We only get one really good chance to hit herd immunity.
But - longer term - agree with you - wouldn't it be awesome if we had full-viral scans of travelers, international or otherwise - would shut down flu pandemics as well (most of them are imported from other countries).
Most nations aren't getting the vaccines yet. Vaccination distribution and delivery require complex logistics. Nations that don't have a good public healthcare infrastructure need time to gear up. Vaccinations have just started and are being given in a phased manner. Blocking International Travel (or at least mandating a 14 day quarantine) helps keep variants in check.