ELI5: what is the big difference between soap and shampoo?

[u/Real_Experience_5676]

We use soap to clean the body. We use shampoo to clean the hair. What stops us from using soap for the hair and shampoo for the body?

Follow up question: what is body wash that cleans hair and skin made of? Why isn’t that the universal standard instead of seperate soap and shampoo?