It's not just that they are afraid. No one has succeeded in building production-level systems at scale purely using "formal methods". There are a variety of way

It's not just that they are afraid. No one has succeeded in building production-level systems at scale purely using "formal methods". There are a variety of ways to measure how a system compares to a spec. For instance, you can measure throughput at various points in a system. However, formal methods are much more detailed, and when they are applied today, it's usually within a limited context as a way to verify or do model checking.