Why wasn't MacDuff a captain?

https://preview.redd.it/r5orjxq7svif1.jpg?width=1024&format=pjpg&auto=webp&s=628ca2e78bde7e771a7a56d4aaa008991371f7be

I always wondered with this episode:
- The aliens wiped everyone's memories and planted MacDuff as XO. To carry out the captain's orders. Why didn't they simply make him Captain? Then everyone would be inclined to follow his orders. By making him second in command they make him one step lower. Why not just hack the Enterprise computer to show he's the Captain?