I don't think I've ever seen a convincing answer.
Logically these are the possibilities I can see:
1. God doesn't exist.
2. God exists but is powerless to intervene.
3. God exists and could intervene, but he doesn't care about human suffering, perhaps he set it all up and then got bored with his new pets?
4. God exists and could intervene, but this would be against the rules of his grand experiment in which humans have to suffer because ( insert convoluted reason here ). Though we might wonder why he gave humans the capacity to steal, murder and wage war in the first place, why didn't he design humans to be much nicer, more caring, less selfish? We might also wonder about why God designed humans bodies so they would get cancer and all those other diseases.
5. God exists and chooses not to intervene with a good reason which human intelligence may not reach easily.
Genesis warned: The tree of knowledge is something that you choose to eat from it, the same day you shall surely die. It simply says that your intelligence is not reliable when used to judging God.