Reality has a lot to do with it for folks who believe there are absolutes. As someone who subscribes to radical subjectivity (ie Truth(tm) is unknown and unknowable), I don't tend to agree that anything is absolute. Our thoughts and even languages have biases, etc. Concensus about ethics and morality therefore changes as we evolve. Reality in a culture is what we agree it is (ie society is a 100% artificial construct and can be different). Should it be? I'd say yes, because power (exerted by various interests) has distorted reality, negated consensus, and is making (among other things) ethical positions impossible to see clearly (and even then one individual's views or actions cannot represent consensus).