Re: What about boats, buses, cars, train, horses?
Correct. Anybody asked how the weapons used in France reached Belgium? I don't believe they were carried by a passenger in his or her air luggage. They could have come by sea, or by land routes.
There is a lot of illegal trafficking using sea transports.
Along the Adriatic Sea there's a lot of illegal activities to move illegal goods across it - drugs, weapons, and so on. Drug dealers recently are using small planes able to land using uncontrolled strips and fly low to avoid radars. Illegal immigrants and goods enter even using standard ferry boat lines, hidden in the many lorries going to and from Turkey, North Africa and East Europe... bribes always help.
Also, for an organized group with enough financial support should not be difficult to obtain counterfeit documents and enter from countries that raise less alarm. Or are they going to require every airline in the world to handle data about every flight?
Do governments really believe terrorist can't switch using untracked routes if needed - and if they already don't?
Maybe politicians are so used to travel by plane only they can't believe someone could use any other kind of transport...