Face it, people say "I could care less" for the same reason they say "then" instead of "than". There's just something about the English language that makes most of its native speakers unable to use it.
You came up with this irony theory because you've made the same mistake yourself, and your ego wants to deflect the accompanying shame.
The word "not" is not a particularly big word, but then again, most native speakers can't use "than" either.
It's amazing what an outburst of theorizing wankery your comment sparked.