The rules prevent it other than in exceptional circumstances. See my comment above about the consideration of an award to Gandhi for example. Apart from wanting the candidate to continue to do good, there are also considerations such as practicalities of where the money should go as the money is awarded with the intent of using it to further the purposes of the prize, though the recipient has wide latitude, and so it is not a given that it'd be appropriate to just hand the money to someones estate for example.