The offer of an early return was rejected because mission/operations plans required/expected a ship to return now. If this ship (which has been docked at the ISS for months) had returned early with the astronauts, then NASA would have needed to pay Elon to launch another as replacement so it could return now. Might as well leave the astronauts for a few months and have them do some science. It’s just more efficient and less wasteful than paying Elon twice.