Not really. It is more that no one really thought that this would be something you had to design for.
Now, a bomber hit the Empire State Building at it survived. So, you could design a building to withstand a plane hitting it. The problem is that the Empire State Building is heavy; it is probably the last skyscraper whose design was controlled by dead load.
There had been studies into failure of buildings after the Oklahoma City Bombing; some of the fruits of that research led to designs which were installed in the Pentagon by 2001. However, for most buildings, it wasn’t considered to be worth it. This includes skyscrapers both in the USA and around the world.
A * B * C is generally considered below the cost of making most buildings plane impact resistant, so they don’t do it.
The B-25 bomber and Boeing 767 airliner are two very differet aircraft.
The WTC towers and the Empire State Building were also very different buildings.
So there events are not really comparable.
The point was that New York City saw a large plane hit one of its largest buildings. The failure mode was known. It just happened to be that no one cared to design for that failure mode later.
The last plane crash into a building in NY before 2001 was back in 1946, when a transport aircraft from the US Army Air Forces flew into 40 Wall Street.
That was 55 years before 9/11, and 20 years before construction of the twin towers was started, it was believed to be a solved problem, which in context wasn’t that unreasonable.
The other thing is, both towers were plane impact resistant. Both of them took dead square hits from airliners and remained resolutely standing afterwards. What it turned out they were not proof against was an ongoing raging inferno inside that was hot enough and carried on long enough to weaken their critical structural elements.
If the planes had not been laden with fuel and/or if it had not ignited for whatever reason, the towers probably would not have collapsed. They probably wouldn’t have been readily repairable, though, so then the question would be what to do with two massive skyscrapers with giant holes in the middle of them. They’d probably have to be demolished eventually anyway. Said demolition would have killed far fewer people.
