The 72 virgins is actually more of an esoteric principle that a number of literal virgins. Many Muslims dont know that and they never will.why do Islamic people believe in 72 virgins in paradise?
The number 72 is of great significance when dealing with Semitic philosophies of religion and spirituality.
72 represents the number of specific influences through which God's action/influence over all creation.
Islam came out of Judaism (Muhammed was a Hebrew/Jew at one time, but got fed up and decided to find God for himself).
So if you want to understand the significance of the number 72, you may have to search about:
"The 72 Names/Attributes of God".
But essentially, to see the 72 attributes as virgins which one will receive, represents the idea that one will "become one" with the powers of God.