Several reasons. 1. There is no hard evidence for the existence of God. None. This is one of the main ones. 2. Irregularities in Quranic and Sunnah morality; Mohammad married a child, committed genocide, spread the faith by the sword ergo cannot be the perfect carrier of a perfect message from a perfect God. The Quran also allows men to have sex slaves (ma malakat aymanakom). The Sharia punishments are also inhuman imo. Amputation, flogging, stoning... no thank you. Hijab also oversexualises girls at a young age. 3. The gross injustice in religious reasoning. A serial killer could repent on deathbed and go to heaven, yet an atheist who is charitable and commits to his community is damned to hell for eternity for simple disbelief. Your actions do not matter as long as you repent and believe. If you don't, you're screwed. You could be the most noble and generous person in the world but you're still screwed. 4. The scientific errors. Adam & Eve as the Quran tells it is biologically impossible (I'm a biology student.) The Quranic account of creation completely goes against the theory of evolution which is supported by 1000s of pieces of evidence. Too numerous to list here. The embryonic development is incorrect (bones and flesh develop simultaneously because they are formed from the same germ layer, the mesoderm. The Quran states that the bones develop first and are then covered with flesh.) besides, it's basically copied and pasted from the Greeks! Look up Galen. Camel urine is a medicine, mountains secure the earth, etc etc I could go on. 5. Religion divides people and has not provided us with real, material progress since its conception. (PS I'm aware of the Islamic Golden Age, blah blah blah. Yes, there have been many great Muslim scientists, but Islam itself has not aided humanity in any concrete way. 6. Terrorist groups. Come on. You can call them extremists, but how come there are so MANY extremists and ALL of them misunderstand/misinterpret the Quran? And ALL of them interpret it in the same way. 7. Islam is the only religion that has a phobia. Go figure.