It’s a religious specification and the degree of what needs to be covered varies. Most Muslim homes are dominated by men who set the standard based on their interpretation of what the Koran says.

“Niqab consists of covering up completely, including gloves and a veil for the face - leaving just a slit for the eyes, or covering them too with transparent material.
Muslim scholars have debated whether it is obligatory to don the niqab, or whether it is just recommended without being obligatory.
There have also been more liberal interpretations which say the headscarf is unnecessary, as long as women maintain the sartorial modesty stipulated in the Koran
There are additional Koranic instructions - seen as ambiguous and therefore much debated - for women to draw the "khimar" (or scarf) to cover the "jayb" (or bosom/upper chest), and for "the wives and daughters of the Prophet and the women of the believers to draw their "jalabib" (or cloaks) close round them".

Religious and cultural traditions vary across the Muslim world, stretching from Indonesia to Morocco.
But it may also be left to the Muslim woman to decide for herself, whether she wants to cover up fully with the niqab, as an expression of her faith and Islamic identity, or not.
In countries such as France and Turkey, where there are legal curbs on religious dress, it becomes a matter of women's human rights to wear what they want.
But at the same time the niqab is such a powerful statement that more liberal Muslims sometimes can be heard objecting to it, especially in more developed societies, where women have fought long and hard to shake off restrictions seen as outdated and imposed by men.”
“According to Islamic scholarship, hijab is given the wider meaning of modesty, privacy, and morality, the word for a headscarf or veil used in the Koran is khimār (خمار) and not hijab. Still another definition is metaphysical, where al-hijab "refers to the veil which separates man or the world from God."
Since the 1970s, hijab has emerged as a symbol of Islamic consciousness "and an affirmation of Islamic identity and morality" in opposition to "Western" values. Muslims differ as to how "hijab dress" should be enforced, particularly over the role of religious police that are enforcing hijab in Iran and Saudi Arabia.”