Because the body's changes do not always correlate to modern society's views on marriage laws/age of responsibility.
The female body enters puberty at the age it does for evolutionary reasons, probably. The lifespan of humans was relatively short less than a hundred years ago, so to engage in successful procreation, it had to be done early. People mentally matured much earlier back then because there was no choice - life was hard, so the ignorant bliss of childhood wasn't encouraged for long.
Society is a lot different now though, medical advances and better diet have expanded the lifespan for people living in developed countries. We have a better understanding of how the brain develops as well and the consequences of that at each stage of life. We're also overpopulated to the point that we don't need lots of young girls pushing out babies to save the species, so now girls are considered actual human beings with more value than their reproductive capabilities.