Currently, the body design is completely in your hands. You don‘t get to control the creature‘s movement, so at least this way your design input is still essential to the success of your creature. This makes the whole simulation more interactive and interesting from a user's perspective.
That being said, I totally agree that allowing the creature designs to also be optimized automatically would be a very interesting feature. Similar simulators that do include such dynamic body changes can yield quite impressive results, so I might consider adding this feature in the future.