Complaint After Complaint

Why do people say this? This is one of the many common errors made by people that just really eat at me because I feel like people should realize that it makes no sense. If your response to something that is frustrating you is to say “I could care less!” this implies that you care to begin with. If you are trying to express that you genuinely are not affected by something the appropriate expression would be “I couldn’t care less.”
I would have thought common sense would lead most people to this revelation, but apparently not. Now you know.