You need to decide what you care about. If the plastic part is important to you just use any bar soap. It’ll clean your head just fine. However shampoo is meant to not only clean but moisturize and protect your scalp so it often has oils and stuff in it. The oils is what makes it runny and not really able to maintain a bar shape. You could just as easily use a bar soap and add scalp moisturizer after you shower but that would just be another plastic bottle.