Religions are similar today, after thousands of years of existence. That doesn't mean that they were similar when they began. In the ancient world, the question of "what happens after death" was a very important one, and there were so many opinions floating around. So early religions were anything but generic
If you were to start a new religion today, you would definitely have to have something unique. Nobody is going to pay attention to your religion if you regurgitate what the others have been saying for thousands of years.