Religions are almost complete systems. That's the utmost reason for them to sustain longer than other idolatrous concepts. Religions has given the answers for all the desired problems a person which he may occur with. No matter the answer, it is made believed by the owners (probably not by the authors) of the religion. But since the reality is not staying perfectly streamed, and new knowledge is ever expanding across the traditional believes, religions must come out with magical concepts.
So there are so many crappy concepts about life, death and the stage in between. The results of good and bad and reincarnation with the knowledge gathered in the previous lifetime. Do better and go to heaven, otherwise to hell. A person's disease identified as a result of a sin he has done and so forth.
Lot's of people are there to believe them. Disregarding the fact that all the crap there, (Though there are some lot of good stuff people are not aware of...) people are believing what ever the owners say about it.
Is it because people are not self confident to live and learn from life. The ups and downs and goods and bads?
Or are they blinded by the magical theories of those systems?
Or is it because they are lazy to use their brains a bit and get out of it. Do they think that staying at their cage is better than coming outside...
What ever, the ultimate reason of a living thing is nothing else than living itself.
So what the hell!
You smoke, you die.
You quit smoke, still you'll die.