The "problem" is that it would obviously show that the impact of torrenting is much less than one lost sale per download. And reflect that even the downloads that are lost sales are offset to some degree by the people who download to sample and then buy when they otherwise wouldn't have, and the free promotion from the people who downloaded it talking about it with other people etc.
The result is that the likely "negative impact" would be small or could even be positive, which isn't the narrative Hollywood wants to spin. Because they actually want DRM for entirely different reasons than that, specifically because it allows them to control content delivery and thus prevent independent content from being displayed on equal footing with big studio content.
For each movie, copies are released over time and a correlation can be looked at between quality of rip, distribution by file sharing, and box office receipts.