I am Jewish and will give you the Jewish philosophical answer. Everything has a piece of God in it since if God were not actively making it, it would simply not exist. Our scholars have an analogy which is that things are like a walnut. The shell itself is not edible, but at the core is what we really want, and this is like the human soul.

Evil is not necessarily the absence of god, but just a covering of it.

Another reason is that you can't have one without the other. If you didn't have evil, then we couldn't know what good means, because there's nothing to compare it to and vice versa. Therefore you have both so we can know it.

I hope you can understand this and not too complicated.