[LeanderK]

I think it's that most, or nearly every code, is actually not critical and failure is ok. I would argue that code for every part of facebook is actually not critical. Of course it's bad if there's an error or the feed of instagram is actually not correct, or you can't send a message right now. But it's not critical, facebook will survive and the cost of eliminating those bugs is just not worth it. After all, unit tests only reduce the possibility of failure to an acceptable level. Amazons cloud had outages and amazon survived. It just about economics. The cost of hiring the right developers, writing and maintaining the code etc.

Writing code is not about solving a problem perfectly but delivering solutions under economic constraints.