Obligatory mention that the more correct and logical formulation is "could NOT care less".
Languages are full of phrases that get used so often the original meaning gets lost organs others that are heading that way. Raining cats and Dogs is ancient and nobody is quite sure where it came from, but as long as people understand intent there’s no need for to add up correctly.
Words with opposite meanings can end up converging. ‘That’s cool’ and ‘that’s hot’ sometimes have the exact same meaning depending on context.
But the correct formulation is a shrugging question "I could care less?", with the implication that the care-meter is already pegged at zero.