Because cancer can only be prevented during the phase when the cancer cells are contained within the epithelium and have not spread more deeply.
When we talk about precancerous lesions, we mean lesions inside the epithelium (intraepithelial lesions). Under the epithelium there are blood vessels and lymphatic vessels.
If the cancer cells spread under the epithelium, they enter these vessels and travel to other areas of the body, where they ‘take residence’. In this case we are talking about metastasis