The reason why crypto might be even harder than the other engineering tasks you mention is that in those the "adversary" is indifferent, not actively trying to exploit every loophole you might have left. You might build a bridge that has some small weakness, but no-one will come and stack weights in the exact pattern that exploits the weakness and makes the bridge collapse.
Beyond that, failure of a bridge or an airplane is not likely to go unnoticed. Every failure will be investigated, documented and studied. Crypto failure by contrast can just easily be silent, deadly, and continuously unnoticed for extended periods of time.
"The second relates to the design of structures. It is time for engineers and architects to get together to devise new structural forms that offer a higher degree of protection not only against terrorist attack, but also against other hazards. There is much to be learned from what happened in Nairobi and Dar es Salaam, in Oklahoma City, and at the World Trade Center. Similarly, retrofitting of existing structures needs to be studied systematically, as it can reduce, at modest or virtually no cost, the potential for damage."
