The problem is mirrors don't have a certain appearance, they change texture based on what's in front of them and because of that the brain doesn't have a static model of it so the brain just mixes everything you've ever seen in a mirror together while trying to render it

Some people see just normal things some people see nothing and some see real fucked up shit like their face getting torn apart or something. If you panic while looking at a mirror in a dream, your brain knows what you fear way more than you

Another thing to consider is the human brain being also able to render really high quality images and animations.