If God is real then why doesn't HE do anything to stop it?
HE DID. However, to understand WHAT God did, you need to understand WHO God is.
God is perfect. God is holy. God's very nature will not allow Him to be anywhere near sin. Therefore, God cannot be anywhere near us.
Anyone who has ever lied, cheated, stolen, hated, gossiped, lusted, and so forth does not deserve or have the right to be close to a holy, perfect God.
That is why the Bible says that the natural penalty for sin is separation from God or death.
For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord. Romans 6:23