Unless you need a kidney; then we just bury perfectly good ones regularly, and let the donated ones get a bit more stale while we confirm the opt-in eleventy times.
It's because of the US Constitution and voters' very firm, consistent, coherent stance on bodily autonomy. Even if we consider it to be costing a life, your say over your body is considered absolute, and no federal, state, or local government is allowed to pass legislation that influences what happens to your body.
Just kidding! It's all determined on a case-by-case basis depending on the most conservative perspective of the dominant, favored religion.