Why does God create a genome that can't improve? Well if He created one that was in need of improving, then you would be asking why didn't He make it better? Why bother with a coding system? because the fact that there is an intelligible system that we can observe allows us to conclude the designer's existence. If God designed each being individually, then there would be no observable coding system that demonstrates an intelligent creator. Why does the genome degrade? It has amazing capacities to maintain DNA's original structure. There are many enzymes whose purpose is to fix mistakes and mutations. It degrades when we stray from natural environmental conditions. We eat carcinogenic foods and don't exercise. Proper nutrition and physical activity increases the transcription of many enzymes and helps our cells to take care of their genome.