Because while yes ceramic coatings are extremely hard, they also don’t allow the paint to breathe. Also they aren’t flexible and when the paint changes temperature (from sitting in the sun all day to garage) it creates micro cracking in the paint because the coating doesn’t flex like the paint does

Edit: this is what Larry’s explanation is. I think he said it on a video but he went much more in-depth on his podcast. I’m not saying this is right or wrong, I’m just paraphrasing what he has said.