First, sites like Geocaching.com strictly forbid it as part of the terms of service. Right there it's technically "illegal" to pull data off their site and reformulate it for our use.

Our statistics are also "Open" in the sense that you can track any/all geocaches regardless of their type. Terracaching, Navicache and Geocaching.com are the major listing sites, however many "bonus" caches, or unlisted caches could never be counted as we would have no place to "spider" to get that cache information.

It's also very unlikely we'd be able to tie all of them together accurately.  In order to make the process work, you'd have to use the same login or screen name across all of the sites we spidered.  Perhaps a database of the sites/screen names could be maintained but considering the number of users and the amount of data to link together, it would be cost prohibitive to do such a thing. 

Our stats are contributed by the community, the accreditation system makes those stats more reliable than before.  We look at KeenPeople.com as a place where users can contribute their statistics and keep them all in once place.  While it may not be convenient for the end user to update their stats, it does give you the ability to track them all for all sites in one place.