> It puzzles me why this isn't the case for software
In my anecdotal experience, its because corporate software projects are not typically run by people who are good at building safe things - but rather, just building things quickly.
There's a huge issue with the mentality of "it works, ship it" being propagated.
I build systems software for safety-critical and mission-critical markets, and I can say without a doubt that if there aren't at least two quality stages in your process workflow (and your workflow isn't waterfall), then you're going to be in for a rough time, rookies.
Always, always delay your releases, and always, always, eat your own dog food by testing your delayed releases in your own customer-like environment, which is to say, never release a developers build ..