Because science. I believe in documented, evidence based conclusions. Big Bang, evolution, dinosaurs etc. The idea that it was all created by a man in seven days flies in the face of all reason.
I think it was useful back in the day when we needed something bigger than ourselves to govern our behaviour - the fear of hell, the promise of heaven. There needed to be a set of rules (Ten Commandments) and a reason to follow those rules. So it served its purpose.
I think it has outgrown its usefulness at this point in society. We have the law, we have a much deeper understanding of people and the world we live in. I think if you need the promise of a reward or the threat of a punishment in order to do the right thing, you're not doing it for the right reasons. A lot of the teachings are outdated and encourage harmful 'traditional' ways of thinking/living, I've seen first hand the effect on mental health growing up in a very religious household can have and the physical effects some religious activities/beliefs can have. I think we need to move away from it as a society in general.
However I do recognise the comfort it brings. I have a friend who cannot fathom the concept of death, he absolutely can't handle the idea that he may no longer be here one day. For him, I understand why he needs to believe in an afterlife. For my poor auntie who lost her daughter too soon, I understand why she needs to believe there is a heaven where she will be reunited one day. Far be it from me to challenge those coping mechanisms.
For me though, I quite like the idea that I am simply stardust. I take much comfort in the idea that I am a tiny, meaningless speck in time and my life on this one small planet in an infinite universe matters not. I like the idea that I will simply go back to the earth when I die. I don't need any bigger being or plan.