This is a pretty difficult problem to solve. Facebook would need to delete your data from production databases, database backups that were made while you had an account, caches and CDNs, etc. But even if they managed to delete the data in every possible place they could find it, a third party might have made copies of your data via Facebook's APIs some time in the past, and they might still have it.
So, even if Facebook offered such a feature, I wouldn't have much faith that it actually worked.
I'm not making a moral point about blame and culpability here. I'm making the practical point that it's impossible to delete all of the data if nobody knows who has it.
Yeah it's really going to suck for FB (boo hoo), they just can't win. It's years past due. The warnings have been on the horizon and they've only amped up their data collection to the point it's become even more unmanageable than it already was.
They have to do this anyways when you choose to delete their account. And according to Zuckerberg's testimony, he is very confident that this functionality works very well currently.
It's a fundamentally important problem to solve. If to get rid of my old and no longer wanted public activity record I have to sit for hours upon hours manually deleting stuff, I'm going to be much more judicious in how I post in the future.
Also, it's Facebook. They can figure it out, they just don't want to.
So, even if Facebook offered such a feature, I wouldn't have much faith that it actually worked.