If it were implemented as a "convenient" export button, then it would offer a "convenient" way to easily steal all your data. Leave your computer for a second or accidentally forget to log off at the library or apple store? Anyone can come in and download your entire history and pictures and chat logs and who knows what else.
"that's not realistic because it would take too long" or "just ask for password again when you export". OK, but now you've made it even more enticing to steal someone's password since you've created a super easy vector to then gain access to all their info, possibly nicely organized as well. In this sense its kind of nice that its pretty difficult to get old info off of facebook currently.
Secondly, what counts as social network data and what doesn't? Should amazon be forced to provide you with an export of all your purchases so that another web store can offer you as good suggestions and prevent lock-in to amazon? Arguably amazon (or any web store) has already proven to better use this data, and it's as real or more real than facebooks data. How about my Starcraft stats and battle.net chat logs? I know, let's have a long series of litigations to figure this out, in the meanwhile creating a scary and perhaps prohibitively expensive proposition to any new startup that plans to use your prior behavior/data to enhance your future experience.