The problem is that fundamentally, to Facebook, a user's login account is a separate thing from the user's profile. You can delete your account, and with a straight face Facebook can assert you've deleted it. But your profile, the mass of data and content that you put on Facebook with your account, and all the data associated with it through their social graph and algorithms, that never gets deleted.

In a very real sense, "you" still exist in Facebook. That's why when, weeks, months, or years later, when you login, Facebook recognizes you. You create a new "account", and Facebook very conveniently associates everything it knows about you (which it never forgot) with your new account.