They're Raging Hypocrites
A lot of people have a strong interest in portraying terrorists as dedicated, evil badasses. Hollywood, is always desperate for proper bad guys. There are people who are trying to get you to vote for their terror safety laws or sell you terror safety equipment. And of course, there are the terrorists themselves.

When they convince us that they're dangerous Bond-level supervillains, we have to deal with minor airline travel annoyances. But more importantly, we are sending a message to people who sympathize with the terrorists' cause that, "Hey, this works! They're very effective guys!"

Governments and financiers will continue to see the terrorists as movie cops -- sure, they may be loose cannons and have to turn in their badges every couple months or so when they blow up some civilians, but dammit, they get the job done. And look so cool doing it!

 Most Of Them Are Nincompoops
But while those are particularly fantastic screwups, they aren't the exception, they're the rule. Just like most tropical storms fart themselves out before they become a hurricane and get near land, most terrorist-wannabes appear to step on rakes or slip on banana peels or something before they get anywhere near a target.
One group of suicide bombers got together for a group hug before they presumably gave the team cheer, signed each other's yearbooks and went off to martyr themselves. Only they hugged too hard .

They're Self-Destructive
On top of all that, let's take a look at what they were trying to accomplish in the long term if they ever got their shit together. Bin Laden told his followers to stay away from small attacks and try to go for a big one, on an important date like July 4 or September 11. The reason that all their terror attacks hadn't driven the U.S. out of the Middle East, he reasoned, was because none of the attacks were big enough.
That's right. He thought if they could just blow up a whole city or something, America would go, "Oh man, too rich for my blood! I don't want to play anymore!" and bring all our troops home. Just like after Pearl Harbor! Or September 11! That's exactly what America does every time someone lands a massive, horrible attack on us!

They're Raging Hypocrites
So, OK -- they may be an incompetent bunch of man-children. But, at least from the perspective of a pious Muslim who thinks America is the enemy, they mean well, right? They're at least really sincere about wanting to get their countries back on track with good Muslim values and fight Western corruption.
Well, no. By now you've seen Osama bin Laden's compound. While it was architecturally a piece of shit, it was luxury living in Pakistan, in a neighborhood popular with retired generals and other Pakistani elite. His last known videos showed him roughing it in a cave, which is how he wanted his followers to think of him. He was asking them to live on bread and pious fervor, running from cave to cave, so he couldn't very well tell them he was living in a nice house with satellite and Internet and three wives.

They're Mentally About 12-Years Old
Another reason it's really hard to maintain any respect for these guys is the retarded ideas they come up with. One enthusiastic terrorist wannabe published an online propaganda magazine where he suggested covering a "tractor" or "farm vehicle" with swords and I don't know, driving it slowly through the enemy I guess. Osama bin Laden objected to this, but because it was too barbaric, not because it sounded like a jihadi equivalent of, "Hey, what if we had a dinosaur with a railgun mounted on it ... and also Batman is riding it."