Because sometimes, good cannot accomplish what is necessary to accomplish and would only get in the way of the end goal. [Raistlin smirked.] However, there are some times when good is the only solution to the world's troubles. Even then, it may not be the "good" as we are familiar with it.
Besides, good and evil often cast the same spells. Evil merely uses it for personal gain. Or so it has been in my experience.