The introduction of "nearly free" streaming and subscription services have made it very easy for people to steal videos now.
Also whereas its relatively easy to see which sites (and who) are selling still images, the tech for doing the same with video really hasnt caught up yet.
Almost too easy to steal content these days, earlier this year I tried this random free app for stealing video off social media, it was literally too easy, I didn't upload the "stolen" content to any stock site but I think it was 60 seconds and it spat out a perfectly good .mp4.
However, with so little money to be made in stock video and photos now I wonder if there is much interest anymore from the bandits, they want to make more money than a few dollars and cents, probably not worth the effort anymore to steal.
There's that plus the agencies deal our content in bulk to companies in places like Russia and China which don't have copyright laws like we do and it's just a lost cause to keep that content secure.
There is good money in trafficking of stock footage, that's why the agencies are so desperate to deal our content around the world to those "trusted" global partners.