[marginalia_nu]

A compounding factor is that it's much harder to debug code you're not at least reasonably familiar with.

Even if you can gain such intuition through looking at a piece of code, in a realistic scenario, more often than not you don't actually know in which file or method the bug is hiding. What you have is a code base and some externality that is wrong, the debugging process is essentially deepening your understanding of the relevant code (in relationship to what it should be doing) until you understand what is wrong.

Debugging isn't being able to use a debugger or a profiler or syscall tracing tool or whatever, sure they help but the critical skill is being able to quickly and accurately model the behavior of a system in your head, in combination with a tacit understanding of where bugs tend to occur.