Creating a physical collage is not copyright infringement because no copying is involved
If you invoke copyright for arguing, you cannot use a version of copyright that only exists in your head and that you wish would be true.
When collage uses existing works, the result is what some copyright scholars call a derivative work. The collage thus has a copyright separate from any copyrights pertaining to the original incorporated works.
I did not cite a collage to argue that ai are collages. As I said, I cited collages as example how weird copyright law is. Using material for training an ai is currently also ruled as derivative.
And about morality. Well, in my opinion, the morally wrong thing here is not the training of ai. We all stand on the shoulders of giants. But rather the replacement of human work by ai output. If you focus on how ai models are made, your position is vulnerable. The copyright angle does not work. And 100% consented models would defeat that position completely. Meanwhile the ai output of such models would still replace human work.