It's not the same thing at all. Losing an organ always involves major surgery, you have a very limited number of them and there is already an established system for altruistic donations.
Also, taking a kidney is a one time thing you can't defend against if a criminal doctor does it while you're in for a different surgery. That would be a real problem if secondary sales were legal. Compare that to pregnancy where you have months to get an abortion.
Giving birth also comes with real complications, but it's a natural part of life and already very common so we know how to manage the problems a lot better. Surrogacy is closer to any other form of sacrificing your health for money (like working in a coal mine) than selling organs.
Also, taking a kidney is a one time thing you can't defend against if a criminal doctor does it while you're in for a different surgery. That would be a real problem if secondary sales were legal. Compare that to pregnancy where you have months to get an abortion.
Giving birth also comes with real complications, but it's a natural part of life and already very common so we know how to manage the problems a lot better. Surrogacy is closer to any other form of sacrificing your health for money (like working in a coal mine) than selling organs.