When you travel around the world you enter and leave many countries and when you enter you go through immigration, and yes customs, and when you leave you again go through immigration. They check who comes and who goes.
The US is the only country which does not check who goes. The airlines may do they job but they clearly have no interest other than booking the revenues.
Thus a simple solution would be to have immigration at the exit point as well as at the entering point. Then all passports can be scanned in and out, biometrics on non US citizens checked and the problem is solved.
Of course you would need a few other checks but this one would make a great deal of difference.
The US is the only country which does not check who goes. The airlines may do they job but they clearly have no interest other than booking the revenues.
Thus a simple solution would be to have immigration at the exit point as well as at the entering point. Then all passports can be scanned in and out, biometrics on non US citizens checked and the problem is solved.
Of course you would need a few other checks but this one would make a great deal of difference.