John Piper wrote a really helpful article a few years ago on why satan is left on earth. Here’s the first paragraph:
Part of the problem of evil is the problem of why Satan is given so much freedom to harm the world, when God has the right and power to throw him in the pit. God will one day do away with Satan altogether (Revelation 20:3, 10). That will be no injustice to Satan. Nor would it be unjust for God to do it today. So why doesn’t He, in view of how much misery Satan causes?