MIT asks for credit. GPL asks or credit and GPL'ing of things built atop. Unlicense is a free gift, but it is a minority.
AI reproduces code while removing credit and copyleft from it and this is the problem.
Why? The software is still there and you can still go choose to use it.
Another possible consensus would be that copyright don't exist, and anyone can copy proprietary or copyleft work and improve it. Nobody would be harmed in such situation, original author still have its copy. I would have no problem with such state - but it must be same for everyone, not just FOSS.
If a library I wrote was used by BigCo, then I could point to their license file and mention that in a job interview or something. If they have Claude generate something based on my code, they don't put it into their license, I don't get the resume fuel, and my work is unrewarded.
I have gone back and forth about how I feel about AI training on code, and whether I think it's "theft", but my point is that the original code being available is kind of missing the point.