Calcium is in no way exclusive to animal products. There are entire cultures who’ve never consumed cow’s milk that don’t have a higher incidence of osteoporosis than the developed world. Producing cow’s milk required humans to learn how to domesticate animals, which was achieved relatively recently in human history. So it is illogical to think that humans evolved to require nutrition from a fluid that they could not get in nature until centuries later.
Humans are mammals. Like all mammals, we consume milk during infancy, and after the weaning process, adults do not require their mother’s milk. If we really needed milk afterwards, wouldn’t it make more biological sense to continue drinking milk designed for our own species? If that sounds strange, consider that we’re drinking milk from someone else’s mother, and not even from our own species.
Good vegan sources of calcium include dried herbs, sesame seeds, figs, tofu, almonds, flax seeds, Brazil nuts and kale. Most vegan milks are fortified with calcium, so we could just consume those as we would do any cow milk.