Why is the human body so crap except for the liver?

[Epistemic Status: Speculative unifying theory of biology.] I don’t know about you, but I greatly resent having to be a biological organism and subject to all the poor engineering and design decisions that entails. This has many manifestations. A recent one is often wondering, why is the entire body such garbage except for the liver? Take the kidneys. Once you reach adulthood, they start to slowly decay. You can damage them through not-obviously-dangerous stuff like taking too much vitamin D or taking ibuprofen while dehydrated. Significant damage typically leads to scarring and a permanent reduction in function. Or take the gums. If you brush too hard or don’t floss enough, they may retreat down your teeth, never to return. Most of the body is like that. But the liver. My friends, the liver! If it’s injured, it will usually heal without scars. As you age, it typically maintains near full strength. You can give half of your liver to someone else and it will regrow to full size and…