Once a girl reaches puberty, these hormones take over and she starts to have boobs, curves, and becomes more emotional and feminine etc. These hormones also enlarge her hips and uterus. This lining builds up in the uterus and sheds itself every 28 days if she doesn't get knocked up, thus she gets her period.
People tell me, "Well we have periods because we're supposed to have babies!" But even if this were true, I read this study that a woman ovulates 400 eggs in her lifetime. Who could actually have 400 kids? Even in the old days when they had like 12 kids and they only lived to be like 60, a woman would still have more periods and pain than she would kids.
Now it's the 2000s and nobody has 12 kids anymore, some people don't even like or want kids, but we're still stuck with periods and the average age of menarche is declining due to all the calories and stress. As a result, we've suffered too much from periods: we miss school, lose blood, have cramps, waste money buying "feminine hygiene" products, and basically go through a lot more than the guys do.
Periods are flawed.
They were always flawed, even in the ancient days.
Too much blood loss and waste.