Why is it difficult to understand what a theme is

Theme is the topic of the story.

Explanation:

It can be so difficult to understand what the theme of the story is.

Here's some key ideas to understand what the theme is:

You can look for the main plot of the story, take Percy Jackson for example in the book Percy is learning about his father who is the God Poseidon. It's hinting at Greek Gods, which is literally the entire theme of the books, it talks about Zeus, Hades, etc. All of which are Greek Gods if you're like me and study the gods.

Look for the things the author writes more often than any other things, find key words, etc.

Hope this helped :)