Because in OP's case, they indicate that the company either doesn't care that OP wrote software the company uses and thus has already demonstrated competence an

Because in OP's case, they indicate that the company either doesn't care that OP wrote software the company uses and thus has already demonstrated competence and then some, or they indicate that the company's hiring process is too inflexible to avoid insulting someone who they already know is beyond qualified. Either one is a red flag, and a perfectly great reason to bail from an interview. If hiring/HR is rotten, the rest of the company is likely similar.