> Instead of airplanes as weapons that could be used to kill thousands...
As pilots had been screaming for for years prior to 2001, cockpit doors have been reinforced and locked, and cabin procedures have changed so that those doors are rarely unlocked. [0] This happened shortly after the WTC bombings that totally destroyed the buildings. This means that the only people who can get control of an aircraft are the crew of that aircraft.
The first of the two things that stops "another 9/11" had a one-time monetary cost and was done twenty five years ago. The second was the murder of everyone on board three airliners by hijackers. Prior hijackings were a diversion and an annoying delay. Now that "hijackers will kill you all" is on the table, hijackers will be outnumbered ten or thirty to one and will not succeed. Remember that the plane intended to crash into the Pentagon crashed in a field somewhere because its passengers heard about the ones that hit WTC and -correctly- determined that they had absolutely nothing to lose.
> ...terrorists can blow up a few hundred people [at a checkpoint].
The fact that this doesn't happen indicates that even a jetliner with a flimsy, unlocked cockpit door wouldn't be used as a weapon to kill thousands. Murder -let alone mass murder- is vanishingly rare. By and large, people simply don't want to kill or injure each other... they just want to go about their business.
To support this point, I'd like you to tell me when was the last time you heard about a cell-phone-battery-bomb? You may remember those from back when Israel's military booby trapped the batteries of phones that they believed were going to be delivered to many members of the opposing military... and then set off those bombs all at once. Shorty after the bombings, Bunnie Huang [1] did some thinking and came to the conclusion that not only is it pretty easy to fabricate such remotely-triggerable bombs with like five-figures [2] worth of perfectly ordinary equipment that is entirely legal for anyone to have, if one built such a bomb with the explosive force of a hand grenade it would be [3] undetectable to a TSA CT scanner operator, and -because batteries are sealed- entirely undetectable to explosives swabs.
If The Terrorists Are Constantly Out To Get Us, why haven't we seen these bombs? It has been what, three years or so since those very clever booby traps were distributed to and used against opposing forces? Where are the domestic bangs and booms?
[0] One expects that airlines did not wish to do this because its cost was greater than zero. I wonder how the cost compared to the destruction of three airliners, two skyscrapers, all the deaths and cleanup, and twenty five years (and counting) of security theater.
[1] Huang is a fellow with fairly substantial commercial electronics design and fabrication experience
[2] USD
[3] ...in part because of the variance in battery construction from manufacturer to manufacturer and even from model to model...