Lack of knowledge and skill. Nor is problem-solving an innate skill. It must be learned, practiced, refined, and practiced again. People develop habits, many of them dysfunctional. Among the most severe from the standpoint of problem-solving is rigidity.
Rigidity is a type of mental block characterized by fault-finding negativism.
Another form of rigidity is selective observation. That is the all too common fault of seeing only what we want to see, hearing only what we want to hear, and ignoring everything else that does not conform to our view of how things should be. And, because it has been learned, it can be unlearned. In the past, and to an alarming degree in the present, public and private education tend to suppress the natural desire to be creative in one's approach to the solution of problems. In later life, such tendencies must be unlearned.