Jesus came to take away the punishment for sin, but he didn't take away sin itself. Sin still exists. God allows us to make choices, and not all of our choices are to obey God above all else. That's what free will is. When all of our disobedient choices get together, they contribute to the total picture of sin in the world, from the beginning of time.
Some people call it systemic evil.
Racism is a systemic evil, and so are sexism and world hunger. Still, it's important that we try to do our part and work for justice in Jesus' name. It's a huge task, though, and we'll need Jesus to finish it for us. The book of Revelation has lots of images in it that theologicans have tried to explain for years. Most of them are directly addressed to the early Christian church, but several images are full of meaning for people of all ages.