They write about it for at least two reasons: 1) It is a problem that has perplexed and affected every person that has ever lived, and 2) atheists and others falsely believe it to prove the non-existence of the Christian God.
And for those two reasons it needs to be written about, although there has not likely been anything new written about it for a long time. It's just that different viewpoints or the same viewpoints stated differently can help one in understanding and dealing with the difficulties that arise.
While we cannot say why evil exists or how it came to exist, we can say more than that. We can say that God provided the means, through Christ's death and resurrection, to defeat evil and that at some point it will not exist.
I don't think that that is the case. It could be argued that God is most interested in our happiness but that in order for our ultimate happiness, we need to be purged of sin through the process of sanctification. Without holiness, no one will see the Lord (Heb. 12:14), and that is where our greatest happiness will be found.