The world is God's to do with as He pleases. He created it and has the right to destroy all of it or part of it if He so desires. As matter fact, the world as we know it today, He will once again bring to total destruction some day.
"The world is God's to do with as He pleases. "
The Earth is under a curse because MAN chose to fall.
And because of this the world was marred.
And man has to live in this world and witness the effects of sin.
Simple.