Why do people still consume cow's milk?

That's a load of nonsense. Humans are unethical in countless other ways, but it doesn't make our exploitation of dairy cows any less heinous, and it doesn't mean we can't work towards solving all of those injustices simultaneously. In fact, stopping dairy consumption is probably one of the easiest fixes humanity can do, with some of the most far-reachingly good effects for the world in general - as I've already posted, the meat and dairy industry contributes to world hunger, climate change, and the prevalence of heart disease and cancer.

The idea that we should be "allowed" to exploit dairy cows because we are "at the top of the food chain" is similarly nonsensical. The dairy industry is a profoundly unnatural system to which the rules of biology do not apply. What's more, biological systems like food chains never have any moral weight to them - they describe what does happen in the wild, not what ought to happen in human society.