Why isn't the world perfect? Sin.
God intended the world to be perfect. It says in the Bible God created the world in 7 days. He created Adam and Eve and gave them his whole creation, a perfect world, with one rule. The rule was to obey God and to not eat from one tree in the Garden of Eden. Then a devilish, terrible serpent named Satan tempted Adam and Eve with sin. Satan told them that the only reason God didnt want them to eat from the tree is because the fruit would make them like Him and know good and evil. That is not true. Satan lied to them, and Eve ate from the tree and gave her husband some. They disobeyed God, and He punished them. That was the start of sin and the 'Fall'.
See, the world was perfect until sin came. They had a choice to obey God or sin, and they chose to sin, basically ruining the world. God's punishment was death. In the beginning, humans were supposed to be immortal. In earlier times, people like Noah lived 700-1000 years. Now the average life expectancy is nowhere near that number. It also says in the Bible that God grieves in his heart every time we sin. We hurt him so much all the time, and we think so little of it. He sent his one and only son Jesus to die on the cross for our sins. He loves us so much even though we hurt him. It says if you believe God's son Jesus died on the cross for us, you will go to heaven.
Anyways, the reason the world isnt perfect is because of us. Sin. Our own fault for making the world bad. God punished us and we deserve it.