Because religions are just stories.
And bad stories at that. I was brought up as a Buddhist, and the canon was full of old, tired tropes. Reincarnation, supernatural powers, heaven and hell, the whole gamut. The story of other religions are no different. The heroes just have different powers and the miracles come in many varieties. To be honest, I fell for the Buddhist stories until I entered university, and this fact still embarrasses me to this day.
Some would say that religious stories teach us something about the good life. But the same can be said for other kinds of stories. You can learn a lot from Aesop’s fables, Harry Potter, and the Lord of the Rings. Also, consideration of the fact that other people are conscious beings capable of feeling pleasure and pain, the same as I am, helps many decisions. I don’t need to believe in fantasy stories in order to become moral.
Believing in stories do not give my life meaning. On the contrary, religions would have us believe that the present life means less than the hereafter. But all evidence shows that the present is all we have. I find meaning in family, friends, science, music, arts, and many other human endeavors. Throwing away religion would not deprive me of anything.
Religions are stories, and they should be treated as such. So that’s why I am an atheist.