Why is it with animals when they get really sick and there is no cure and they no longer have a quality of life the humane thing to do is euthanize them, but when an actual human gets that way they must continue to live a horrible existence until they stop breathing "naturally"? Why can't they have option to decide if they want to continue to live or die.

I know here in America assisted suicide is illegal and can possibly come with severe penalties including jail time. Doesn't seem very "humane" to me.