There are two main reasons for this at the moment.

1. It‘s just easier for me to implement this way. Dynamically changing each creature‘s body throughout the simulation would require me to change large parts of the current code and spend a lot of time trying to handle all of the edge cases - of which there are a lot.
2. 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. I will try to figure out the best way for me to incorporate this into the simulator, but given it‘s complexity it‘s a rather long term goal of this project.