It's deleted because all connections (and meta data) has been erased.
In a database, if you null all fields but keep the entries and their inter table relationship intact, you still have identifiable data up to a certain point.
Imagine you have data of specific people and you only have one person per country, having a user and country relationship, if you null a single data of a specific user, you'll still be able to find the user just by analysing to which country that user was connected + some external information.
This is the classical problem of deriving personal data from statistical reports and why data anonymisatiom is so complex.
