Because people need hope, love and guidance. Because we were born as flawed creatures, made ready for change, for love, for heartbreak. Life was designed to be a roller coaster ride between the good and bad, to learn lessons, to learn to have compassion.
I do not beleive in angels or demons. I do beleive in good and bad, evil and morally gray, people are fluid, they can be have a smattering of all three within themselves, and a terrible, atrocious incident that they have performed can eventually bring them back to the redeemed, by being put in prison, facing themselves. Then there can be the everyman, who is always good, right, just and one little thing can tip their scales, force their hand or switch them to the path of the unjust. We need all manner of people, we need faith, hope in order to grow and evolve. Life is an ever swinging pendulum, life always seems to sway us one way, and not before long it begins to sway toward the other.
We we're given a gift, a life and a planet to look after. Sometimes we have done amazing feats with those two things, and other times we have desecrated them, ripped them asunder, crushed them under our boot heel. It is nature, it is chaos, and because of chaos, both must be personified, good and evil.
Some people personify the good as Angels, who are virtuous and stalwart, a force to be reckoned with. Demons can be worshiped too, since evil is more centered upon the self, there are always struggles with inner demons, or something that is at its core diabolical that people worship, aim to achieve, aim to take.
I think people beleive in them not because they are always necessarily real, but they need a beacon to believe in, to model themselves on. Who is to say which is right, or more correct? I cannot, but if it gets a person through the day, on either side of the fence, the belief alone if sound enough.
I hope that makes sense, it made sense to me, anyway.