How is writing some website with rails/whatever even slightly comparable to deploying hundred-million-dollar hardware designed for a combat environment with lifetimes of 30+ years?
In most startups, failure doesn't result in death.
If you're talking about naval vessels in combat, people are going to die no matter what. If you make LCS up-armored to be more survivable then you buy less of them, and the ones you do buy go slower and have a deeper draft which means they can't go as near the coast, which in turn completely changes their military effectiveness, which can then lead to even more deaths somewhere else down the line as a result of enemy action that strong-LCS couldn't interfere with.