Everybody lies on exit interviews. You mostly don't want to burn bridges and frequently will need some references. So HR departments have 0 useful information as to why people are leaving and even less about the costs of having developers go.
> Everybody lies on exit interviews. You mostly don't want to burn bridges
Hold on - not everyone does (I offer myself as anecdata). And I am more than happy to burn bridges if I'm leaving a toxic workplace because, well, it's toxic.
Finding a job with higher pay or a better work environment makes the rest of that team (and possibly members of other teams) reconsider staying.
Personal anecdata: one person leaving a team of 10-20 caused 3-5 other people to either leave or re-negotiate in private. In extreme cases, this triggers a chain reaction.