Because not everything is equally good. What we call evil is not something in itself but is the absence of something, the absence of good.
Think about height, is there something that does not have height? everything has height, some things more than others, but everything, after all, has height.
No, good actions would still be good.and have meaning and value in themselves, even if there were no evil actions being done.
But how do you know that something is good if there isn't evil to compare it with?
Or in other words: If everything is good, good has no meaning.
Because not everything is equally good. What we call evil is not something in itself but is the absence of something, the absence of good.
Think about height, is there something that does not have height? everything has height, some things more than others, but everything, after all, has height.