Shampoo, especially anti-dandruff shampoos, are designed to strip away oils built up in your scalp. This is great for your head - where lots of natural oils are produced - but not so good for your face. Your skin microbiome is a lot more sensitive, drier, and more prone to becoming off-balance, than your scalp. Essentially - scalp hair is oily. Beard hair is coarse and dry. Shampoos are made to remove as much oil as possible - terrible for beards. Result: brittle hairs, split ends, and that tight, itchy feeling.