Simple…
Because of men and their desire for power and material things.
Ask yourself, if there was no evil inside our hearts, would we be suffering? Will your neighbor envy you to the point of wishing you bad? Will your partner be so jealous he/she threatens your life?
This life is a test from God.
We have all these bad feelings inside (jealousy, envy, greediness etc.), we all come to life with it.
The test is: what are we doing with these feelings? How are we responding to situations whether we control them or not?
Those who successfully master these feelings and dominate them, pass the test. Those who do not succeed, they fail (yet they have the option to succeed if they sincerely repent).
The prize? Jannah (Paradise).
Or Jahannam (Hell).
Either way, we end up choosing where we want to go, not God.
So, suffering in this world is caused by men only. Unfortunately.