Because the Koran is quite clear when it says that you'll be "a new creation in paradise," and that you will be "recreated in a form unknown to you," which seems to me a far more appealing prospect than a virgin.
For Koranen er ret tydelig, når den siger, at man vil være "en ny skabning i paradis" og at man vil være "genskabt i en form ukendt for dig," hvilket synes en meget mere tiltalende udsigt end en jomfru.