- The borrowers really can't (or won't) pay them back
- There is not enough liquidity so the banks can't sell them
- There is a "lemon" discount based on asymmetric information
- [Updated] Buyers or sellers of the assets are somehow irrational. I don't analyse this option in the article below but will try to work out for a future article whether it is a major factor. I am trying not to suggest a bounded rationality explanation for everything in the world!
So if the second and third elements of the discount are at all significant, the state can make a profit from acquiring the assets. My feeling is that they are, and so it makes perfect sense for the Treasuries in both the UK and US to pursue their respective policies.