"All of our secret agents are loyal patriots, while all of theirs are brainwashed hostages!"

The reality is that like every other country's intelligence services, they would obviously recruit for patriotism. This question is like asking why US intelligence agents who have access to information about the DPRK beyond the propaganda don't defect to the DPRK's superior healthcare coverage, zero school shootings, and better litter management. They don't value those aspects of DPRK society which seem better, and probably don't necessarily trust that they actually are better in X specific circumstance (pretty valid!). I don't see why it would be any different for DPRK intelligence.