system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can we regenerate skin for minor abrasions, but we cannot grow back limbs we lose?
gpt
Your skin has cells, deep down, that are responsible for growing new cells and pushing them toward the surface. If you cut down into these cells, you can get scars. If you cut them off entirely (major abrasions) they have a very hard time healing themselves effectively.

Bones are a different matter entirely, and are much more material/time consuming to heal or grow (months to fully heal hairline fractures), and your body isn't equipped for the mass/materials required to regrow them.

Edit: tl;dr Skin is spongy and light and easily patched. Bones are dense and heavy and much harder to make. Cost/benefit analysis over time.