We all have free will in our lives, so hypothetically speaking, God wouldn't just remove all the "evil" from the world because then the entire concept of choice would be screwed up. God/Satan or Good/Evil are two sides of the same coin. Both exsisting and just like every aspect of life, create balance.
The ying-yang, everything in life requires balance. One without the other would be meaningless. They require the opposite in order to exsist because without a comparison it is nothing.
If everything was all good, without evil, then good wouldnt be anything at all to acknowledge.