To quote one of the comments
To the Americans, Afghans are treated worse than their dogs. Afghan’s lives means nothing to them.
People were huddled onto the wheel bay doors and other exterior parts of planes. I don’t know how panicked you’d have to be to try something like that that would almost certainly kill you.
The airfield is a crowd of people. There’s no way to prevent people trying to stowaway or hop onto the exterior of the plane as it taxis and starts takeoff. I don’t know if the pilots even would be able to tell.