Because much of the world is ruled in ways that promote the interests of one group over the interests of other, more poor groups

Because there are alot of nasty people out there who like to cause it! There will always be misery as people do not always agree with eachother.

The existence of evil in the world is only a problem if you believe in an omnipotent, omnipresent Deity who is dedicated entirely to good. This is known as good vs. evil dualism. Not all religions view the nature of the Divine or Deity in this way.
For instance, I see Deity in a construction vs. destruction dualism. Sometimes destruction and endings are necessary to remove obstacles that have been constructed. For instance, the destruction of an abandoned building is necessary in order to construct a new building.

We often get attached to things that really need to go in order to make room for new opportunities, etc in our lives. Think about this, if everyone followed God's commandments then how much pain would there be in the world? Maybe a little bit from hostile weather? And still then people would be helping out so much that things would be better much faster.
If we dont experience the pain, and misery then how will we know the difference and how will we be able to enjoy the pleasure of life?
World population is currently growing by approximately 75 million people per year. Is this your misery ?

http://www.etext.org/Politics/AlternativeOrange/1/v1n4_mmpc.html
http://en.wikipedia.org/wiki/Overpopulation