The answers to this question are many and I think include the following:
This world is under the curse pronounced after the Fall of Adam & Eve. The curse includes physical and spiritual death, disease, pain, and the sweat of the brow principle. All of nature is included in the curse.
Satan is the ruler of this world. He is a genius but is totally arrogant. He would like to bring about perfect environment in this world and get everyone to ignore God and worship him. But he cannot control the world in its entirety, eliminate the free will of man or the gospel message. So the curse continues, the world is not perfect and Satan, smart as he is, cannot fix everything or eliminate the curse.
Every individual is born with the sin nature. Does this cause suffering? You bet!
People make wise and unwise decisions. We reap what we sow. We create our own personal misery though bad decisions.
But there is also suffering for blessing. Not all suffering is because of sin or bad decisions. Only in this life can you experience suffering. In Heaven, suffering does not exist. So as blue_ladybug pointed out, suffering is used to train and teach us. One of the privileges in this life (believe it or not) is to experience suffering and get to personally see God’s grace sufficiency.