> and everybody says 'its not me!' and it doesn't get fixed.

Test automation is not a remedy to prop up broken leadership. I would appoint an arbitrary owner of the defect and that person would visit with other people as necessary to remove or dismiss various functionality from blame one by one. Once the appropriate collision of changes is discovered it will become more clear how to address resolution.

...and they would do that by writing individual unit tests.

Your development process should not allow someone to merge a change that fails the test suite, nor one that is too large to reason about.