If we assume that there is no afterlife, what reason do we have to comply with a person's wishes as regards treatment of their corpse? In particular, it is striking to me that we should respect a person's wish not to extract their organs after death; what reason could we possibly have to heed the wishes of someone who no longer exists, especially when the donation of their organs could literally save the lives of several people?

A further consideration is that, given that many people have strong wishes -- whether rationally grounded or not -- that their corpses and probably those of their loved ones be treated in certain ways, it would be highly upsetting to many if they were to become aware that such treatment quite possibly wouldn't be provided. In other words, even if you can't harm someone after they are dead, you can harm the living by treating the dead in ways of which the living disapprove.