The reason is, God has not allowed it to. God didn't plant people on earth so that they could just wipe themselves out. And when He sent Jesus to rescue us, He didn't do it with the idea that Jesus would be ineffective among the people.

God is Great. He fills the whole universe and beyond. God wants a certain number of people to be with Him in glory. And it's going to take a lot of people to satisfy Him, this "Guy" who fills the whole universe.

Since most of the people don't want to have much of anything to do with Him, He needs more and more people just to get the number that He wants. It's the percentages. It's the numbers.

The world will be around until Jesus returns. After Jesus has collected His own people for God, there will be a new Universe for them, and only then will this old one pass away.

By the way, the return of Jesus could happen anytime. So be ready.