Why doesn't God just destroy evil? Why hasn't He already?
Well, He could wipe everything out except for one thing.
He laid out in the Bible what He was going to do after the rapture of the church. Since that hasn't happened yet, there are at least another 1,007 or more years of history to live through and He always keeps His word, no matter what.
So evil isn't going away because if it did the prophecies that He gave us about what was going to happen wouldn't be able to be fulfilled.
What we can do is try to minimize the evil in our own lives. What is evil anyway but the absence of goodness and righteousness? As we let our lights so shine before men, we banish darkness both in ourselves and in the world around us (Mt. 5:14-16). Evil may not be completely destroyed for a millennium or more, but we can work on destroying it in ourselves right now (Col. 3:5-17)