Why does Islam permit polygamy?

According to global statistics, the male and female birth rates are almost equal. However, it is scientifically acknowledged that the rates of females' survival are higher than those of males. Moreover, in wars more men get killed than women. It is also scientifically proven that the average age of females is bigger than that of men which, consequently, leads to increasing the rate of widows globally than that of widowers. Thus, women will come to constitute the majority of the world population and that is why it is not practically appropriate to restrict every man to one wife.