Bombs also aren't really found by airport security. Every bomb that has blown up a plane, has gotten past them. I've never heard of a bomb being detected before it was used at an airport.

Such things aren't necessarily widely reported. A successful bombing gets massive coverage. Thats the thing about security. Most people only notice when you loose. I'm sure there is data out their that justifies the investment. It would be interesting to see it. Wikipedia is not too helpful here...

RE: US News Article | Reuters.com