The thing about shampoo is that it is in the final analysis a soap usually with oily conditioners included. All of that leaves residue. The suggestion was made to me years ago by a girl who cut my hair and it works great. When you put the shampoo in your hand put a couple tablespoons of baking soda with it and wash your hair. The baking soda softens water and causes the soap to rinse cleaner.
“oily conditioners included. All of that leaves residue.”
That residue is what makes your hair shine and comb smoothly.
I was told the same many yrs ago