If a terrorist could bring down a plane by clicking a button on a website, we would see hundreds of planes downed every day.
If it takes a day of preparation, we'd see perhaps a dozen per day.
If it takes a week, we would see another order of magnitude fewer.
If it takes a year of training and preparation by one or more high IQ individuals with several hundred thousand dollars, we'd see almost none. This is why we haven't seen any more 9/11 style attacks even though they were so amazingly effective as a terrorist tool: it's become sufficiently difficult to pull them off after counterterrorism efforts were amped up.
In other words: criminals can still break into my car if they really tried, but I still lock my car doors.
It was sufficiently difficult even before security theater and airline liability reduction measures were ramped up, which is why we didn't see 9/11 style attacks before 9/11.
There is no more incentive for someone looking to cause mayhem to blow up a plane than there is to blow up a shopping mall or any other building with more than ~180 people inside (the approximate number of seats in a 747).
Mainly, though, people feel more vulnerable in a plane, so an aircraft bombing is higher impact.
> If it takes a day of preparation, we'd see perhaps a dozen per day.
Nope. People are deterred by socialization and innate biology. I could take a gasoline bomb to a nearby school or church and kill hundreds of people within the week, for little expense and a good chance of getting off scot free. So could most people. It is not difficulty or risk that prevent mass killings.
A: the number of people radicalized into anti-American terrorists
B: the opportunity that As have to cause damage
I suspect A went up in the late nineties and continues to be high-ish today. B went up with OBL and the Saudis funding the game, but has gone down post-9/11 for a variety of reasons, including decreased Saudi bankrolls, increased airport and aircraft security, tighter restrictions on student visas, and a weakened Al Qaeda.