system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are spacecraft parts both extremely fragile and able to stand up to tremendous stress?

The other day I was watching a documentary about Mars rovers, and at one point a story was told about a computer on the rover that almost had to be completely thrown out because someone dropped a tool on a table next to it. Not on it, next to it. This same rover also was planned to land by a literal freefall; crash landing onto airbags. And that's not even covering vibrations and G-forces experienced during the launch and reaching escape velocity.

I've heard similar anecdotes about the fragility of spacecraft. Apollo astronauts being nervous that a stray floating object or foot may unintentionally rip through the thin bulkheads of the lunar lander. The Hubble space telescope returning unclear and almost unusable pictures due to an imperfection in the mirror 1/50th the thickness of a human hair, etc.

How can NASA and other space agencies be confident that these occasionally microscopic imperfections that can result in catastrophic consequences will not happen during what must be extreme stresses experienced during launch, travel, or re-entry/landing?

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EDIT: Thank you for all the responses, but I think that some of you are misunderstanding the question. Im not asking why spacecraft parts are made out of lightweight materials and therefore are naturally more fragile than more durable ones. Im also not asking why they need to be 100% sure that the part remains operational.

I'm asking why they can be confident that parts which have such a low potential threshold for failure can be trusted to remain operational through the stresses of flight.
gpt
It's not that the tool damaged the computer, but the tool violated the pedigree for the computer.  Since the pedigree is required to launch the computer, it would have been very expensive to disassemble the computer, test every part, and assemble it to be **sure** that no damage had occurred.  To be 99.9% sure that nothing bad could have happened isn't sure enough to pass launch criteria.

The Hubble mirror is an interesting example.  The mirror was made extremely precisely, albeit wrong.  That allowed it to be corrected for later.  There was a plan to test the Hubble mirror, but the schedule was compressed.  Then the Challenger Disaster delayed the launch many months, but NASA didn't want to spend the money on the Hubble test, because they were worried about their budget because of the disaster.