Except that most major software you use has only limited automated tests that are nowhere near comprehensive. For example large parts of the Linux kernel are o

Except that most major software you use has only limited automated tests that are nowhere near comprehensive. For example large parts of the Linux kernel are only covered by manual testing and some automated bug finding tools, not traditional unit or even integrations tests. The answer in practice for most software is that the testing simply is not thorough, bugs are missed, and regressions slip in (and get fixed once noticed). And that is all not necessarily a major problem. Of course, having some level of automated testing, even if somewhat cursory is highly advisable. At the very least least for any smaller components that are easy to test. But a comprehensive test suite for each component is not at all a hard requirement of software development.