At Facebook-scale the data is massive -- far bigger than anyone here could possibly comprehend and that includes the Facebook and Google-ers lurking around.
Data has incredible inertia. And when there's a lot of it, in a lot of different places, I can imagine that it becomes very difficult to keep track of.
I'm glad that Facebook's data export tool included some things that maybe it didn't expect to.
The GDPR prompted them to make the data resurface, so it's not impossible to track this data given a few months of warning. It's just that Facebook as a company does not have an interest in deleting data they collected.
Facebook has a responsibility to protect user data they have collected, and if they can't then they shouldn't.
Oh, most certainly they do want that video. Their business is knowing who we are and what drives us, so they can target those ads better. That's what makes their shareholders money.
That the people working there are human beings who might consider it immoral to keep deleted material, is what most people rely on when using such services... but being kind is not Facebook's goal.
Every application of any complexity has features which inactivate, but don't delete data. At Facebook scale, deleting data is non-trivial, and it would be impossible to immediately delete something.
We all have bugs, including extremely critical security bugs, availability-threatening performance bugs, or many other types of bugs. It's strange that we accept those bugs as merely bugs, without assuming a backdoor, or intentional sabotage, but when it comes to personal data, suddenly it's a nefarious plot. It's an odd position to take that Facebook is not only saving these deleted videos intentionally (for what, exactly?) but that they'll now lie to us and pretend to delete them, but only remove it from their Download Information tool.
Kudos to Facebook for even having such a tool.