In my time, I have potentially written code that some legal jurisdictions might classify as a "crime against humanity" due to the quality.

Not to mention, a team member is (surprise!) fired or let go, and no knowledge transfer exists. Womp, womp. Codebase just gets worse as the organization or team flails.

Seen this way too often.

There's only very niche fields where closed-source code quality is often better than open-source code.

Exploits and HFT are the two examples I can think of. Both are usually closed source because of the financial incentives.