Why is a developer ("coder") not the ideal person to do requirements engineering — and how can developers still help?

Because RE needs special know-how most engineers lack, and a developer suffers from "creator blindness" — they can't see the gaps in something they built. But developers still help: they know their code, how to change it, and the consequences of those changes.

It's tempting to assume the person who writes the code should also gather the requirements, but the two jobs need different mindsets:

• Special know-how — eliciting, negotiating, and documenting requirements is a distinct skill set (communication, conflict resolution, moderation), not a coding skill. It's genuinely hard for normal engineers to do.
• Creator blindness — the person who built or designed something is the worst-placed to spot its missing or ambiguous requirements, because they unconsciously fill the gaps with their own assumptions. RE has to overcome that blindness, which usually means a different person with an outside view.

But developers are a big help, not a hindrance:

• They know their code and how to change it, so they can tell you what's cheap vs. expensive to require.
• They understand the consequences of their actions — the downstream impact of a requirement on the system.

Tip: This is why RE is typically done by a Requirements Analyst / Business Analyst (an outside, problem-focused role) with developers as expert consultants — not by the developers alone.