Anni time of the day: Because Muhammad did it, under Islam by Law, women are merely property, to be bought and sold, however many, and for however long, as the Muslim male wants and can afford.
We used to participate in the holy wars carried on by the Prophet and we had no women (wives) with us. So we said (to the Prophet ). “Shall we castrate ourselves?” But the Prophet forbade us to do that and thenceforth he allowed us to marry a woman (temporarily) by giving her even a garment, and then he recited: “O you who believe! Do not make unlawful the good things which Allah has made lawful for you.”