I'm reading a book on Christianity and I can't seem to find a satisfactory answer to this question:?

Because while God is a merciful God, he also is a just and righteous God. Due to his perfect righteousness, he cannot even look on sin. And due to his perfect justice, there must be a consequence for sin. In order for his mercy to reach us, an atonement for that sin is required in order to bridge the gap between him and us.