The scab on the lips and mouth is different from other parts of the body. Moisture plays a large part as well. If a scab gets wet, it will soften and scrape off easily. When it dries, it hardens again. In the mouth, it would never get the chance to be dry, so it would not be able to form as it normally would. That is why the tissue turns white when wet. It won't scab because the enzymes in your saliva are designed to break down organic matter. This includes food as well as tissue in the mouth which is shed as part of the regenerative cycle. The lips border the inside of the mouth and the scab forms on the outside part of the lip and the white soft tissue is inside the lip. Your mouth is permanently wet so it won't form a hard scab but just a protective layer of soft white skin over it or a dry scab while it heals for 30 days or longer.