The major benefit to implanting it under the skin, as we do with pacemakers, is that doing without permanent holes or tubes through the skin reduces infection risk.
Consider also the danger of having something dangling from your body that is powered by your arterial blood pressure (from a major artery, as the kidney is). A trip and fall could be instantly fatal.
I had a kidney stent in for a few days while I healed from an operation. Let me tell you, it’s a huge quality of life downer (obviously the medical operation outweighed the life style concern). You definitely don’t want medical equipment dangling on the outside of your body if you can help it.
Consider also the danger of having something dangling from your body that is powered by your arterial blood pressure (from a major artery, as the kidney is). A trip and fall could be instantly fatal.