In the United States alone, about 2 million people travel each day in and out of our airports. Our system of air commerce and travel would come to a standstill if we essentially pushed out security to the highways outside an airport. The same holds for the metropolitan transit systems. Yes, they can be made safer with security checks of passengers, but a 20-minute train ride would then become an hour or more. And the public, despite the threat of attacks, would simply not tolerate this.
The reality is that there are just too many soft targets in Europe and the United States to ensure complete security. It is the way our system of freedom has been built.