It's actually very simple. There is no such thing as objective morality. However, social norms are explained by people acting in their own rational self-interes

It's actually very simple. There is no such thing as objective morality. However, social norms are explained by people acting in their own rational self-interest. For example, I want to live in a society where I don't get murdered. Therefore, I want to live in a society that criminalizes murder. That's all there is to it. That is your "Why?" answered. To go one step further, the why of questions like "Why don't you want to get murdered?" are because I come from a long line of organisms that didn't get murdered because of a high drive to not get murdered. The ones without that drive got weeded out. There's still no morality involved, I just axiomatically don't want to get murdered.