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.