Narkissos, for me this is THE question. (and one that is invariably papered over by people who always miss the point of the question by making all manner of unwarranted assumptions)
I've heard...
"Because God has no free will. He had no choice." (reminds me of the answer a physicist gave as to why there was something instead of nothing..."Because nothing is unstable.")
The answer then creates a bizarre cascade (we cannot have free will either then) ultimately eliminating the existence of sin and evil as being simply a wrong way of looking at events, but this snakes back around to the usage of the term 'wrong' because lacking free will, it makes no sense to refer to wrong or right.
I've also heard...
"Because God wants to experience death and the only way he can do this is vicariously."
I understand that some jewish sages suggested that creation was so that God could exist and experience the world that he created (which makes no sense)
I suspect that any question that asks "Why" in this case where there can be no eternal "Because's" (no existence of actual infinities, only potential infinities) will be faced with a definitionally irrational reply. No rational answer can be given because a rational answer is a finite series. At the end we're faced with a God whose action of creation seems to be one that can never be understood in any logical sense, nor would I image that it would be possible to speak of it. I think it would just have to be experienced. I often thought that reconnecting with the transcendent in this way would be the only way possible to "experience an answer". Once that answer was experienced, I would imagine that the answers to all other questions would seem superfluous.
The question of course for me at that time would be... "Is what I'm experiencing real and true, or just an illusion?"