Very true, but the fact that we are not privy to everything does not obviate that which we do observe. We don't have to find a mountain high enough to observe all of creation in order to see that evil exists here, which it undeniably does. Mill would, I suspect, point out that the suggestion of some grander plan which we cannot see being operant is likely to be small comfort to the victims of the evils created by an omnipotent God. "We cannot see how it fits into the big picture" does not affect where the problem begins - with the undeniable fact that evil exists.
I beg your indulgence for the additional observation here that "the problem of evil" can only coherently be a problem for theists:^).
Cordially,