If God is all powerful, He can stop evil. If God is all knowing, He should know that evil exists (and have a pretty good idea of how to use that power of His to stop it). If God is Holy, He should want to stop evil. But God hasn't stopped evil.
Yet.
As a finite species, we have a very limited perspective of time. Most of us haven't even lived a century, and personally, I haven't encountered many people above half that. 6,000 years, compared to our tiny bubble of experience, is a very long time. And so, from our perspective, that "yet" seems trivial at best.
As it happens, the Bible addresses this issue head on. In 2 Peter 3:8-9, we read "But, beloved, do not forget this one thing, that with the Lord one day is as a thousand years, and a thousand years as one day. The Lord is not slack concerning His promise, as some count slackness, but is longsuffering toward us, not willing that any should perish but that all should come to repentance."
From this, we see that our perspective is effectively irrelevant. We think God is being slow to stop evil, but in reality, God is being patient. 6,000 years, to us, is practically forever, but in God's eyes, it's not even a week. No amount of time would be significant to God. But we are. See, the problem with destroying evil is that we are evil. We all think we want God to just wipe out everything wrong with the world until we realise we don't meet His standards either.
The question is often asked, if God destroyed all evil at midnight tonight, where would you be at 12:01? The answer, for a significant portion of the human race, is Hell. Even the ones who answer Heaven cannot do so because they are good, and somehow deserving of Heaven, but because Christ was destroyed as if He was evil on their behalf. And of course, He did the same for you. The problem is, my atheist friends, you have yet to accept that gift. If He destroyed evil yesterday, you would be in Hell. But suppose He waits until tomorrow? Perhaps then, you will be saved. Even if not you, someone among our 7 billion brothers in Adam might be saved, even in the time it takes for you to finish reading this sentence. And if He waits yet another thousand years, that's thousands, even millions more image bearers for Him to bless in the Kingdom.
And so we see that God's refusal to stop evil immediately is actually a result of His patience. But this, too, will eventually run out. Peter continues to warn us that the day of the Lord will come as a thief in the night. It cannot, and must not, be assumed that tomorrow is guaranteed. Whether the return of Christ, or just your run of the mill visit from the Grim Reaper, a time will come when your number is up. Your clock will tick its last tick, your grace period will end, and you will stand before the Lord to give a mandatory account of every thought, word and deed. Even the evil ones. The ones you wouldn't want God to know about. The ones you wouldn't want to be laid bare before the world. The ones you are hesitant to share even with your closest and most trusted friends. Everything you want to hide, everything you wish you could take back, the regrettable things you may even be remembering right now as you read this, they will all be studied rigorously as God decides exactly how to deal with them.
And let's be honest here: You'd rather He didn't. Lucky for you, He would rather not have to either. He loves you. As Peter tells us, He is not willing that any should perish. So what's His solution? The cross. Jesus, the perfect Man, died on the cross for sin. So God doesn't have to punish you. Your sins have already been punished in Christ. Through Jesus, God can just skip the horrible things you've done and move on to considering how to reward the good. But it takes faith. Without faith, Jesus' sacrifice doesn't cover you. But if you repent of your sins, confess Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved. Don't wait! That "yet" is significant in two ways. Significant because time is short to God, but significant because time is precious to you.