system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: if our body cells are all replaced every 5-7 years, then why do we have permanent scars, stretch marks, etc. ?
gpt
The collagen cells that make up your scars can reproduce as well.  There is blood flow to them, etc...

Scars form when you cut the skin so deep there is nothing left.  Typically most tiny cuts form by cutting multiply layers of skin but not all of them.  Skin grows from the inside out shedding the top layer with the 2nd layer every so often.

When you get a scab your body closes the hole above the topmost layer with extra cellular fluid (and white cells) and as the layers regrow they push the scab off.

When you cut too deep the body responds by filling the gap [depending on how wide it is] with a collagen matrix we call "scars."