I don't interview well.
I wrote a program from scratch, tweaked it throughout my PhD, used it to get results for my thesis. I am quite confident in the robustness and accuracy of the program.
I couldn't explain my code to an interviewer (partially because it wasn't production-ready code, but mostly because I don't interview well). I'm glad I didn't get the job. Moved on to work with people who know I'm not a hand waver.