They don’t. Add to the rinse water, and the smell is gone by the time the spin cycle has finished. I second this hack - vinegar does wonderful things to hair as a rinse too. Unless your hair is coloured, where a vinegar rinse wouldn’t help at all. Mine isn’t coloured, so I rinse with a generous tablespoon of vinegar to 1 litre of water after shampooing, and don’t need conditioner. At first you notice the smell, but it’s gone in 5 minutes. The mild acid neutralises the soap part of washing powder and hair shampoo.