When being asked "Why God does this?",why do Christians respond by saying "It's beyond human comprehension"?

Well that is because the answer is beyond human comprehension. We do not know why God has done some things until maybe a long time after. We can only see a little bit of what he is doing now, but God can see everything He has planned and and has done in the past. But there are some things that we can know why God is doing this. Such as if you go to a place you never go and you don't know why, but then someone needs you to pray for them or to lead them to Christ. Well now you know why you felt led to go to this place was to help this person or to give them God's gift of eternal life. I hope this helps!