It's because hair care products are a scam. That natural body sebum is all your hair needs. Most products cause an imbalance on your scalp and you way overproduce as a response, hence hair getting gross fast. It took months of greasy hair before I got to a healthy level. I use soap in my hair at most once a week. It's often more than 7 days before I feel I should wash nowadays. I have over 12 inches of hair for reference.