I checked on this last week. The human body reabsorbs about 2/3 of its endometrium. We, and a few other species like elephant shrews and some of the larger primates, shed the rest in a bloody mess because our early fetal development requires a thicker, larger endometrial layer than the body can fully reabsorb. Generally, mammals from species that have large uteri compared to their body sizes are the ones that menstruate, and this is the most commonly accepted reason why.