Why do people believe the state of the hymen determines virginity?

Historic,social and: Historic reasons. It is generally believed that a female with an intact hymen is still a virgin. This is vitally important in many cultures. There is a mistaken belief that a moman with an intact hymen is pure & has never engaged in any sex. Which ignores the fact that she may have engaged in anal or oral sex or mutual masturbation. Also a torn or ruptured hymen can be "repaired" so again a virgin.