Mainly because the ad placements are determined on the fly and are calculated by a very complicated formula. They've got this magic black box sitting there running the program and they couldn't tell you what it's going to do if they wanted to. Now they can (and should) tell you where your ads were placed in the past but mostly they don't because the list would be ten miles long and would make your eyes bleed.
These networks sound great in theory but they're just not quite ready for prime time. Network distribution of any kind is good for filler, but I wouldn't rely on it as a mainstay either as an advertiser or a publisher. I don't doubt that some of you folks will disagree with me, but that's a different topic.