Truck bombs will continue because they are easy! You can prepare them in a warehouse that you control and then drive them where you require. If you meet a road block that discovers the bomb - you can detonate it early. The driver is in control of the truck at all times.
By contrast, the a/c is beyond their control at all times. The hijacker has to first gain control of the a/c.
For an aircraft, if there is proper screening of the pax, their luggage, the servicing staff and there is no access to the flight deck, then you have covered as many bases as you can.
Banks are not changing their vault doors because they already exist. Over many years they have taken them as far as they reasonably can to counteract the threat. They also know that money can be insured against loss - human lives can be insured against loss but people are rather more concerned about losing people than losing money.
Besides, most financial theft these days takes place at small banks and shops that are not so protected. Once again, it is a judgement between cost of the safe and cost of defending it. In a bank or shop - anyone can walk in and demand money with a gun. They will probably get some of it too, in order to save lives. Comparing banks of money and aircraft of people is not an even comparison. Unless you think that people can be replaced just like printing more money? The big financial theft is electronic.
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