They're missing in the sense that they were never born. In many countries males are considered more valuable than females. Gendercide and Infanticide are common in these countries. Some places in Asia are notorious for it. Males make more money than females and are considered more honorable in certain places. So, people end up killing baby girls in the womb or once they're born in order to still have a chance to have boys.
Left up to nature, more boys are born than girls; however, males are more likely to die during infancy than females. This brings the distribution to be about 50-50. China and India are among the world’s worst offenders of infanticide. According to a study, from 2000 to 2004, there were 124 boys born for every 100 girls in China. In india, for every 1000 boys, there were 914 girls. This is mostly due to selective abortion.