Why do people wearing saris not cover there hair?
Muslim girl and/or women cover their hair through wearing the Hijab because it is a mandatory duty (Farz in Arabic). It is a religious fact. they should start to wear at age of puberty or before. It is a sin not to to wear it. Covering hair is also required by Judaism and Christianity although they are not strictly following it.
Only some Orthodox women wear wigs for modesty reasons. All other wig-wearing in Judaism is not based on religion. According to strict halachah, married Jewish women are required to cover their hair when in public for modesty reasons. Some women therefore cover their real hair with a wig. Other women use hats, berets, or scarfs.