The more likely explanation is that for whatever reason they all decided to film this video on a day Henry Cavill wasn’t around.
However, it’s fairly clear that this video was filmed on the spur of the moment.
One would think they would make arrangements to be sure the entire cast was together.
They’re standing in front of what appears to be a makeup table, which is (we hope) why Ezra Miller is in a robe and Jason Momoa is not wearing a shirt.