Because far too many people believe that human life is the most precious thing on Earth and willingly ending it is literally the worst thing you could ever do.
There's also the worry that it will be abused. Kids who want their inheritance, people who have been abused and told it's the right thing for them when it's not, or just insurance trying to save money by avoiding more expensive treatments
Or even more grotesque, that the suffering for death is a key part to a righteous afterlife.