Nobody can answer the fundamental “why” of any moral framework beyond humans making arguments for what they think works best and is most ethical based on the premises they hold and the kind of society they’d like to live in. Liberals are just honest about this while people who claim to have access to some objective moral code that exists outside humanity are generally just referencing earlier tribes of humans who didn’t know what the sun was and then claiming that this is objectively true and good because they really believe it is.