There are (at least) two meaning for "why ought we be moral":

• "Why should an entity without goals choose to follow goals", or, more generally, "Why should an entity without goals choose [anything]",
• and, "Why should an entity with a top level goal of X discard this in favor of a top level goal of Y."

I can imagine answers to the second question (it could be that explicitly replacing X with Y results in achieving X better than if you don't; this is one driver of extremism in many areas), but it seems clear that the first question admits of no attack.