It is not obvious why selection should favor menopause or the continued survival of individuals that can no longer reproduce. The famous Grandmother Hypothesis had been used to explain the evolutionary significance of menopause. A new study conducted on the Ngogo chimpanzees community of wild chimpanzees in Uganda challenges this hypothesis. Science 27 Oct 2023 Vol 382, Issue 6669 DOI: 10.1126/science.add547
Maybe because most mammals don’t menstruate. Only humans and a few others do, like bats and some apes.
All placental mammals menstruate, most of them just reabsorb the endometrium internally instead of shedding it. Most mammal females are also not in heat all year round.