Our closest worm kin regrow body parts, raising hopes of regeneration in humans
Well, a plausible answer is that if we (or random mutations) re-enable those genes then the end result is simply a lot of cancer.A reasonable default assumption is that each switch in our "genetic configuration" is set to a quite good value.The only exception is if we see a feature that is beneficial at a high calorie cost - those features have been (reasonably) optimized away during our evolution, but would be useful in modern circumstances where food is not scarce anymore; this is a tradeoff where it would make sense to just turn the dial way to the right from where it had been throughout the last million years.