In his book God, Faith, and Reason, Michael Savage quotes another author who makes the argument that God is not omnipotent because he permits all the evil in our world. 
However, the definition of God, “the one Supreme Being; the creator and ruler of the universe”, implies an omnipotent God. If a being can create our incredible universe, such a being would certainly be omnipotent. Also, just because someone chooses not exercise a particular trait does not mean that person does not possess it.
Instead the question we should ask is: why does God not exercise his power to prevent evil? The Bible does give us hints. The book of Genesis records the story of the fall of mankind. Adam and Eve thought they knew better than God. How does God deal with such people? Romans 1 mentions several times that God gives up on people who insist on going their own way and permits them to do whatever they want. The reason evil exists in our world is because we humans allow it; we choose it. God permits evil in our world because he wants to show us the consequences of our actions.
The Jewish physician and philosopher Maimonides maintained the causes of evil and suffering were three. First, we exist in a material sphere. Second, people cause each other pain. Third, we bring suffering on ourselves and this is the greatest cause. 
God has essentially told us humans that if we think we could do a better job in running this world to try it. History shows us how miserably we have failed. God still intervenes, on occasion, in our world to keep things from going out of control, to accomplish his purposes, and to point us in the right direction.
 Michael Savage. God, Faith, and Reason. New York: Center Street, 2017, pp. 15-16.
 Shlomo Pines, translator. Moses Maimonides, The Guide of the Perplexed, Volume II. Chicago: The University of Chicago Press, 1963, pp. 443-445.