You can have perfect airport security (no weapons or potential weapons allowed) but it has nothing whatsoever to do with airplane security.
For example an attempted hijack was thwarted by the cabin crew when three unarmed men broke into the flight deck of a 737 and started to bash the pilots. If those guys had closed and locked the door behind them they would have succeeded. So how could airport security have stopped this attempt? Answer; they could not.
An attempt was made to set alight an airplane using a bottle of petrol (a bottle of aclohol would have worked as well). The perp poured the petrol over the seats and theatened to set it alight. The cabin crew again to the rescue. And what value airport security? None. Once you are airborne those cretins are totally useless.
A successful hijacking was done using a tv remote control ("I have a bomb etc"), and others have been attempted using chopsticks, asthma inhalers, etc. Knives, laptop computers, shoes (and that one was a farce: there is no proof that the shoes would have exploded. Most likely, even if they had been set alight, they would have just smoked. The FBI destroyed them pretty quick, maybe to prevent us from seeing how they were not a real threat), metal belt buckles, cellphones and so on are rarely used and hardly ever (never?) successfully used.
All a hijacker needs is the right attitude and he (usually always a he) has a chance. Taking away the means to fight from the cabin crew and passengers is insane. Creating a mind-set that the authorities, if we give up all our freedoms and rights, will protect us, is the plan and it does not work (well, creating the mind-set is easy, but protecting us in the air by making us criminals at the ariport will never work). The airplane would be completely safe if every passenger had a gun or big knife. No hijacker would be game to stand up. If only one armed person had been on board those airplanes on 9/11 (and I don't mean the idiots who are paid to be air marshalls) it would have been a squib.
The only real result from 9/11 is that it has turned us into cowards.