These people need moral and cultural relativism because otherwise bad outcomes might actually be attributable to a moral framework or a culture.

They also need it so that they can deny that the obvious negatives their worldview brings are negatives at all. John Money based his entire career on it. Example;

"Money held the view that affectional pedophilia is caused by a surplus of parental love that became erotic, and is not a behavioral disorder. Rather, he took the position that heterosexuality is another example of a societal and therefore superficial, ideological concept."

It's complete upside down clown world shit, but when everything is relative and nothing is objective, you can justify anything.