The US has a bigger prison population than the DPRK so why isn't investigated for human rights abuses

So the UN and US claim that there are 120,000 people held in prisons in the DPRk -this is of course an unconfirmed and unverified figure that probably false or exaggerated . The US on the other hand , does have a prison population of 2.1 million nearly 20 times the figure they claim the DPRK has so why is not there a "UN Office for human rigths in the US " ?