"just because you cannot pinpoint the exact art piece(s)"
Precisely, and neither can the owner of the copyright. Unless of course I rub into classical copyright issues ("these two art pieces look too similar") which goes back to my original question, why isn't just enforcing existing copyright law on bad ChatGPT users enough?
If I "use" your artwork, and generate another artwork that looks nothing like yours (either from my brain or by automating it) why do you care?