I think we was just too lazy to implement a method of doing so and saw no reason as to why it would even make a difference in any way, no one deletes information permanently, they just compress it really tightly and say you've been deleted, even if your account has been deleted for 20 years.
With storage becoming ever more cheaper it's pretty much free to keep the data in a compressed format, so all you end up doing is removing your ability to see what you did, doesn't stop them though.