system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If all human cells replace themselves every 7 years, why can scars remain on you body your entire life?
gpt
Human cells can replace themselves, this is correct. But they need a scaffold to replace themselves ON for them to be in the right place. And the nature of that scaffold is why scars stick around forever.

Let's compare our bodies to a multi-floor brick building that King Kong or Cloverfield or Godzilla or something punches a big chunk out of.

You have a couple choices to do something about that building before the weather gets in and wrecks it worse. But a feasible one of them isn't a complete tear-down and rebuild using scaffolding and heavy construction to recreate the building properly. People have got to go on living in there and there's not enough free spending money around to do it.

So you patch that hole as best you can and maybe brick up the opening, and that's good enough for people to keep living in it. But it leaves a not-very-pretty gap in your building. It's functional even if some of the electrical stuff or elevators don't work due to the still missing area, and it looks ugly because you couldn't quite get everything perfect without bringing in super-expensive heavy machinery and shutting everything down, and the bricks don't match. So you're left with a serviceable building with ugly spots that you can't ever afford to make perfect-looking again.

Scarring's the same. The body doesn't have the ability to regenerate huge missing areas because it can't create scaffolding once you're out of the womb. All of the 'heavy equipment' necessary for it is no longer available. This wasn't critical enough of a skill for us to evolve as a species because enough of us survived and had kids even without it to take over the world. So the body goes with a "walling off" strategy without coming with a bunch of perfectly set-up scaffolding to build new clean supporting structures for the new cells to grow back into their perfect original shape. 

And those wall-offs are dead 'hard' tissue that is permanently set into their walled-off shape and can't be replaced. Again, perfect-looking repairs weren't necessary to the survival of our species so we didn't evolve them.