Weak is a relative statement. The thin cross section makes them comparatively weak, and they need to flex, not be static. Witness the Pentagon hit by the 757. Note too the engines themselves - not the mounting - tend to withstand accidents the best.
Yea wings can flex up to 14 feet in flight on some aircraft. Engines tend to survive crashes because they are made of tough materials designed to withstand extreme heat and pressure.
I saw a reconstruction of a crash. The only things I recognized was the four engines. It was a slow speed crash into trees.