how do people in engineering design things?  from previous experience

how do they know how things behave in certain conditions?  from previous experience

I once had to test a certain type of bearing seal called an air seal that exists in the hot part of the engine and supports the power shaft in some turboshaft engines.  The seal worked most of the time, but nobody who had worked with it over years and years could tell me where the high pressure and low pressure areas were, ie, how the seal actually worked.  It worked, that's all.  

A lot of engineering is like that - you design something and hope it works.  You don't necessarily put instruments all over the place and figure out how it works.

Same thing when something ruptures.  Most engineers don't care about anything at the molecular level.  They just want to know under what conditions it will happen.

So the fact that it has never happened before, and it happened TWICE, both times on 9-11, and not anytime since then, when planes have hit buildings before, makes it extremely suspicious and that is what should be investigated, not the 10,000 times or more that it didn't happen.