Doesn´t that mean you the employer failed too? It could be that someone is really not well fit for the role, but it could also be that the employer has no clue how to lead someone to success. As you say
> I'm often pleasantly surprised to see that people moved on and improved themselves
This is for me an indication that the employer lacks critical skills and needs improvement. Being a checklist manager and demanding improvement is the easy part, but making people grow in the direction you want is the hard part.