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because somewhere along the line of our evolutionary path between fish-land animals, land animals-reptiles, or reptiles-mammals we got too complicated for our body to start from scratch on rebuilding shit like bone, muscle, nerve and skin tissue
Look at how our bones brow, we got a few seeds on the tips between our joints and the bone stem it self that grow up until we are 24 or something, it dies out. Bone can glue/repair it self together if its stuck together but that seed is long dead. Muscle is also a hard one, there comes a point when you cant regenerate anymore (think of dr houses leg) when a significant chunk is gouged out but when you're lifting weights, you're ripping tiny muscle fibers that self fix them selfs on the macro which would result on building mass on a macro scale, same with skin