I have wondered about this, people coming in from a perticular country show more infection than the general population (as reported by that country) of that perticular country.
This can be our conformational bias or that immigrating people are more likely to be screened than general population, thus asymptomatic cases are more likely to be netted.
Other than the above two, it also can be that AHU of the HVAC of the airports are compromised. Thus, exponentially increaseing the chances of infection. But, the low point in this theory is that, one cannot be infected by sudden and short exposure to viral load. Although, one tend to spend couple of hours in airport for international flight.
To test the above theory, ground staff manning an airport need to be checked for Covid, as they spend more time in the airport than the passengers.