In part it comes for a lack of common definition of the concept hero.
Research finds that students with different needs look to different types of individuals as a hero.
The concept also grows as individuals mature cognitively to a more abstract idea with a group of generalized characters. Therefore, the traditional American hero of Washington or Lincoln is an answer that you don't get. You could ask why people m might see Lincoln as a hero and students could give you reasons. Remember that young children may have a mixed set of values thinking that money is the most important thing and so focus on rich people. If you want to read more about this and find some lessons folks at Kansas State have done the most recent research and writing of lessons.