If all of our cells die and new ones replace them, then why do we keep some scars for all our lives?

I have an appendix scar from when I was 8. When I was younger it was dark and as I’ve aged it has lightened to my skin color only you can still make it out. The skin there is smoother like the way your lips are different from you facial skin.
So why is it that the cells there never reproduced the same as the old tissue?
The same for stretch marks from pregnancy. Why is it they can fade with time but never totally disappear?