The constantly-changing UI could be considered a way of preventing such tools. Plus, I believe their TOS explicitly states that you can't use scraping tools, and a huge spike of activity would set off red flags. Running it for weeks / months is a massive disincentive to most potential users, so it effectively must complete quickly or it won't be used.

Besides, developing a robust screen scraping tool for a system as large as FB is inherently a very difficult task, and you still need to connect everything you get out, essentially re-building FB. For them to provide an export tool, however, would be ridiculously easy by comparison.