Because to some extent the asset classes are uncorrelated. This is the basis of Modern Portfolio Theory, and the reason you don't just dump everything into the highest-return class (e.g., stocks).
For a simplified model, check out Kelly gambling
For a simplified model, check out Kelly gambling
So with that logic, shouldn't Betterment with its 12 asset classes and 0.15% fees for >$100k invested be more optimal?