I don't think you understand the saying.
lets say on a numerical scale of caring 0-10:
If you are at a 0, you couldn't care less. AKA you could care more, because you are at the bottom.
If you are at a 0 and say you could care less, it is implying that you can drop lower in the scale of caring, even though there is no lower. You have peaked at not caring, this is not possible.
Say you are eating and you are completely stuffed. Would you say "I could eat more"? No, you would say "I couldn't eat more"
The more you know...
