The structure of a problem specifies it; an unstructured problem is less well-specified, and that makes it intuitively more difficult to approach. E.g. summati

The structure of a problem specifies it; an unstructured problem is less well-specified, and that makes it intuitively more difficult to approach. E.g. summation of a column of decimal numbers is a highly structured problem that's very easy to solve. Parsing the speech of yelling drunkards is not so structured, and much harder to solve.