Competence is a comparative value. If we are made aware of someone through mass media we tend to compare them against the top-of-class entertainers, business people, politicians or athletes we usually see represented there - very competent people who have climbed past hundreds of thousands of competitors to heights where the world will notice them.

The pool of would-be terrorists is probably small, given the risks involved. I hope it is. Even if it were not, risks associated with discovery and infiltration probably reduce the efficiency of matching qualified terrorist candidates with sponsoring organizations. In short, when you see some guy bailing out of a flaming SUV in Glasgow, he's probably not the best his home-town had to offer.

Even if terrorists could pick the best people available, training options are limited (unless they get lots of experience setting up bombs in places like Iraq and the Palestinian Territories, of course). Sharing techniques is risky (though the Internet seems to be helping this), and terrorists' short careers don't preserve best practices effectively from generation to generation.

By the time we see a musician they have played in 200 clubs. A pro football player has run 200 scrimmages and 100 games by his rookie year. Each has beat out hundreds of rivals in open competition before we become aware of them. A suicide bomber is probably on his first mission, with probably less than 200 hours of training, and he may have been the only nut case willing to take on this job.

Finally, almost everyone other than terrorists operates in an environment where others will help them succeed. Tough to look good at what you do when just about everyone around you has a big stake in your failure.