You have to come to an understanding that the reason a place is shit is because of this. Revolving doors are actually worse than just a morale problem. Someone quitting isn't just leaving your company, they're taking everything they know about the systems at your company.

My favorite move in tabletop strategy when doing disaster recovery is finding the most important person at the table and going 'Bang, you died driving to work, you can stay but you cannot help anyone'