Some people need a crutch to lean, because they deal with reality of existence. It's an grown ups version of stuffed animal for children.

There are a number of factors at work. First, it gives people hope that everything will come out well for them if they just behave. They believe that god will give them a happy ending. That's so much more comfortable psychologically than acknowledging that sometimes life is bitter, wretched, and short. Second, it absolves them of responsibility. God is pulling their strings.Now this factor depends on the third, which is that god loves them and will be fair with them. So again, everything will be all right. In some religions, they believe they can earn their way into heaven. In others, the decision has already been made, but they still believe that being good will serve as some sort of last minute appeal to the deity who controls ther ever after.
Finally, people cling most tightly to decisions they see as having been their free will. So the reformed smoker is the most adamant against smoking. If someone attacks their religion, to admit the attacker has a case requires admitting they've been duped and are either quite gullible, or just plain stupid. No one wants to admit that.

In a word, FEAR. People don't want to just die and return to dust. They buy into the eternal life thing and they fear doing anything that will impair that possible reward. From day one, religion has been a con job where the sheep of the flock give up their own rights to the "leader" of the cult and yep, religion is pretty much a cult without regard to the over 400 different versions the group elects to follow.

For the same reason any creed or dogma does when applied in place of free will and personal responsibility. It makes life simple. It justifies behavior we know is wrong but takes it out of our hands. But it also allows us to share grief and pain and loss among others. It allows us to take comfort in something greater then just us. Their is too me little difference between mob/group think and religions negative and positive aspects. Between diffusion of guilt in a group and using God/Leader as a excuse for negative behavior. Or using the group as a tool for greater cooperation and support for all. Of course the ultimate failure of these constructs is their need for the "other", the enemy, the heretic, the unbeliever, the stranger, the foreigner. The funny thing is my history with the concept of "God" is very different then most on this site. I have always considered "God" separate from humans acting in the name of "God". The two have nothing in common save a word. For me life here on Earth is very much defined by reason and thought. God is the great unknown of after death and the other unknowns; the far reaches of space, the quantum realm etc. Except I do not think they will always be unknowns. For me I have faith that someday if we can manage not to destroy ourselves we will find "God" out there among the stars, within the atom etc. This faith has a grounding in logic in that human culture and science has achieved so much in so short a time. Religion is one aspect of humanities hunger for purpose. That lack of true purpose and vision I think is a big part of Americas current problems. What will bring us together? Time will tell perhaps a war or two who knows. We are wonderfully insane apes.

Because religion is a manipulation of emotions, not rationality. Get a person addicted to feeling a certain way, or traumatized by horror stories of burning alive preventing him/her from questioning it, and it will be very hard to rationalize those emotions away.

Secondly, a lot of people raised in religion have said religion wrapped around every aspect of their lives. Meaning breaking free of it will force you to reexamine all the things you feel comfortable and familiar with. Including the people in your family.

Likely because it's beneficial to self esteem and hope for the future and because our culture nurtured that

I think everyone started learning about religion while they are so young and it really affects people's mind, even if they get old..

Because they interact with the very crucial grieving process and appeal to subconscious dreams hopes and aspirations.

Psychologically, when people have a problem it's hard to conquer it if they are entwined with the problem. If they are a part of the problem it's hard for them to accept that and find a solution. So, believing in religion is actually a form of externalization. What a problem is externalized, you could talk about it and conquer it much more easily. For example, fear is having an effect on me vs I'm so scared I can't do it. It's more empowering to externalize something and then take care of something.

It's my belief that some people don't have the self-control to get their shit together on their own. Some people need to believe that there's a higher power up there that expects them and wants them to do certain things. So, they do the right thing for the higher power rather than hold the responsibility themselves, and possibly not do the right thing because there is no one holding them accountable but themselves.