Consider that essentially anything asymmetric is imbalanced and less predictable and usually less efficient.

What makes carrying a backpack on your back easier than in an outstretched hand? The weight distribution is asymmetrical in the latter case.

If given the tasks of creating the most efficient thing that rolls on a flat surface, a sphere is unbeatable. Any asymmetry justs adds chaos.

The points about DNA and evolution are nice, but might be putting the horse before the cart. DNA adjusts to what works. Creatures aren't asymmetrical just because it's easier for DNA to code for half.