The absence of heart, brain and courage is symbolic, not actual. In fact, the author wrote this book to see if he could attribute human qualities to non-human characters. So, for the Scarecrow to long for a brain, the Tin Man a heart and the Lion, courage is part of that overall plot. As these non-human characters try to discover their humanity, which the author suggests that they possessed all along, but just did not know how to access, Dorothy helps them as a force for good in the novel.
"The Scarecrow, Tin Woodman, and Cowardly Lion all seek external magic to give them qualities they already possess but fail to recognize."
The Scarecrow is already intelligent, he is the one among the group who figures out how to get out of bad situations, he is awarded a brain by the Wizard, but it is only an external symbol for what he already possesses.
The Tin Man is the most compassionate and caring member of the group, although he believes that he has no heart, he has all the characteristics of a person with a big heart full of love for others. When the Wizard gives him a symbolic heart, the Tin Man feels complete.
The Lion thinks that he has no courage, but courage is how you behave when there is something to fear. The Lion misunderstands what courage really is, the Wizard gives him a special potion that "transforms" the Lion, it too is symbolic.
Dorothy is a force for good in this story, and she opposes, and confronts evil, helping her three friends find happiness.
"The Wonderful Wizard of Oz contains numerous elements that open it up for interpretation. For example, the book has a dominant good versus evil theme, and it is presented in a straightforward manner that is easy for children to understand. From an adult’s point of view, however, the places and characters representing good (a child, the North, the South) and those representing evil (the East, the West) can be fitted into ideological categories."