From my understanding the reason this hasn't been done is two-fold:
1. It's a big research task that nobody has undertaken, until now
2. It's likely impossible to accurately gather stats from the entirety of league history

As to #1, it's surprising that no one at the NFL could find time for this, especially if a couple guys on the internet can, but I think the internet and specifically ease of access to information makes this a much more approachable task than it would have been in the past. But yeah, this is the part that fits with Cheesey and Beast's points about updating historical records when new information is discovered. The events happened, they weren't officially added to the record books at the time, but the raw data has been sitting there in one form or another waiting for someone to do the work.

For #2, this is probably still true and the main reason why these stats currently are and will likely remain unofficial. That said, it will be interesting to see just how deep they can go with the research here. There is game film, game logs, local newspaper articles that are archived and available to the public, etc. I'm willing to bet the data could be dug up in, if not 100% complete and accurate, a relatively comprehensive way.

One thing that is important to mention here is that no record books have been officially updated, Clay Mathews is still, officially, the leading sacker for the Green Bay Packers. This won't change unless/until the NFL adopts this research or undertakes a similar project, and I'm not sure what the likelihood of that is, but I assume it is low.

To Kevin's point re: why don't they update more stats. I think it would be great if someone had the time and ambition to do this. Targets, drops, that sort of thing would be interesting to see. I'm not sure when those started being tallied (and I'm not sure either is currently an official NFL stat). Going further I would love to see some sort of stats for players like offensive lineman, block win rate, for both runs and passes? This is probably hard to tally without knowing the playcall/responsibilities, but some sites such as PFF have done good work in this field.

Originally Posted by: earthquake