Abstraction from consequences allowed for a large number of things to make it aboard planes. With minimal risk of getting caught, you could try once a week to put a bomb on a plane until you managed to succeed.
Shipping containers are effectively the same threat. Due to the globalization and efficiency of trade, I can ship a box full of anything I want anywhere in the world, and the only way anyone will know the contents is if I declare it.
Container cargo is how a ton of bad stuff gets into the country. Not just terrorist stuff, but drugs (and we aren't talking mexican skunk weed here), military grade arms, sex slaves, counterfeit nikes, etc. It is pretty much the wild west.
Something should be done to verify and inspect cargo. A container full of 20 Filipino girls bound for massage parlors should be found and stopped. There is no realistic way of catching more than 1% of illicit shipments without serious investment or impacting the volume of trade that can take place. The best efforts than can be made, unfortunately, are by swinging the terrorism hammer to get dollars.