This is due to two things many times. First, most people don't spend a lot of time in the attic. There are some people who have never even been in their attic! So if you never go up there, you would never find the mold.
The second reason is ventilation. Poor ventilation is usually one of the primary reasons there is a mold problem in the attic. Proper ventilation, after abatement of course, will help to ensure that the issue does not return.