User blog comment:Noctosphere/Why is there still Leviathan in Purgatory?/@comment-4632123-20160429154930

It's possible that Castiel didn't absorb all of the Leviathans, just a large number of them,  with the rest remaining behind in Purgatory. Another possibility is that Castiel did manage to send some back, while the majority managed to hold on to Castiel's vessel.

On a side note, Leviathans did die before Dick; the doctor (forgot his name) was forced to eat himself, Dick Roman ate Victor, and I think Joyce stated she ate some of her previous assistants before she got George. (It's also implied that he chose to eat her after she was decapitated, if I recall)