I think people phrase it that way because otherwise, it would be a much bolder claim to say that your pet theory forms all dark matter, thereby negating all the

I think people phrase it that way because otherwise, it would be a much bolder claim to say that your pet theory forms all dark matter, thereby negating all the other possibilities. There's generally no need to make that much of a sweeping claim, and you probably don't have enough evidence to support it. Why not be cautious?