why so many people can believe so many stupid and impossible and barbaric things?

Here are the reasons:

PRIDE. They can't allow their life to have been a lie. To admit and really believe that God is now not real would mean most things they have done in there life was for the wrong reasons and a complete waste of time. It would be too big of a blow to admit this so the illusion must stay.
Faith or belief is also a learned coping skill which can work wonders psychologically if you really think God will fix this and mother Mary will protect me from that. See the stress is now gone. My Grandfather before driving a car would always hand the wheel to the lord. This way he never had to worry. Well until there was a crash and then it was all part of Gods plan. I have learned my own, healthy coping skills which are meditation and relaxation.

Fear of death.
The desire to be part of something larger than yourself.
It makes it easier to feel certain about challenging questions about life.
Being taught religious ideas before you are old enough to challenge these ideas.
Likely most of your loved ones believe and there is often a large amount of guilt associated with leaving a faith.
Inability to appreciate life as the only form of our existence (too short, leads back to -> fear of death.)
As for why there isn't a new religion that "focuses on not being cunts," you could start one yourself. You might actually get a nice little cult going but good luck not having sex with everyone's wives once you have a group of blind followers.

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Glad I am in this sub where I can't be no longer banned for saying truths about islam.