There are good reasons for using soap and shampoo for specific purposes.
Soap cleans by using two methods. First, the fat in soap seeks out and surrounds dirt. The other substances in soap cling to water and help to wash away the fat and dirt molecules.
But soap doesn’t work well on hair. Because soap contains fat, it doesn’t mix with water and leaves behind a film. On your hands and skin, you don’t really notice this much, and it quickly gets sloughed away with dead skin.
Your hair is another matter: the film weighs down the strands and makes them dull, dingy, and somewhat greasy-looking. It’s just not aesthetically pleasing.
That’s why shampoo is made of a detergent, not soap. Instead of having a two-part reaction, the dirt-seeking molecules in detergents are mixed with alcohol. These alcohol-besotted molecules mix thoroughly with water, and so rinse away completely without leaving a film.
The downside of any product containing alcohol is that if it is used on skin, it tends to dry it out, so soap works best on skin, and detergent on hair.