When I was a kid the only form of shampoo was the super-concentrated sort - it was called soap. Similarly, when I went to sea, unless you brought a suitcase of your favourite shampoo with you (which was impractical) you used soap. Shampoo was almost unheard of at sea.
It was only on eventually coming ashore that I was gradually seduced by the various women in my life to adopt shampoo, as they said they preferred my hair to smell of bubblegum instead of a carbolic soap factory.
Shampoo is the soap company's way of getting you to part with more of your money than is strictly necessary.