Because "cis white male" is used pajoratively by certain extreme left groups like it's a problem or something.

The phrase is fine, but saying it like it's a problem is what makes it offensive. Just like any personal characteristic (age, sex, orientation, race, etc), it's not offensive inherently, it's when other groups discriminate based on it that it's a problem. Just because weird bigot people like Elon are saying it like that doesn't mean he started it