Pretty sure it's more than just choosing not to learn things.
Dictionary.com: Idiot: 2. Psychology . (no longer in technical use; considered offensive) a person of the lowest order in a former and discarded classification of mental retardation, having a mental age of less than three years old and an intelligence quotient under 25.
Being willfully ignorant is foolish. An idiot is too dumb (or mentally disabled) to understand what you're saying or how they are wrong.