Because they’re not copying around images of Joker, they’re copying around a work derived from many many things including images of Joker. Copying a derived work does not violate the copyright of the work it was derived from. The wrinkle in this case is that you can extract something very similar to the original works back out of the derived work after the fact. It would be like if you could bake a cake, pass it around, and then down the line pull a whole egg back out of it. Maybe not the exact egg you started with, but one very similar to it. This is a situation completely unlike anything that’s come before it which is why it’s not actually covered by copyright. New laws will need to be drafted (or at a bare minimum legal judgements made) to decide how exactly this situation should be handled.