Chances are you forfeit the right to sue them over it by signing off on their license agremeent

Regulatory capture has allows AI companies to tell the courts that their AI training consuming any and all copyrighted content is not only fair use but also required for national security, so they are effectively exempt from copyright law. The courts are corrupt, the governments are corrupt, the corporations are corrupt, we've got nothing left to stand on in the legal system.

Copyright is almost impossible to enforce as an individual over items like 3D models and art. If you create something popular it’s going to be stolen and you have to spend all your time issuing takedown notices. And that’s assuming as the other poster mentioned that you don’t surrender rights to your creation when hosting it on one of these services like CG trader.

What I don’t get is are these models not copyrighted by the creator? I’m not one to defend IP law but how exactly is it legal to not have it be opt-in?

I don't understand why you expected this to be different than all the copyrighted creations that AI companies have been sucking up for years without any permission (or opt-in) from their creators or rights holders? Why would 3D assets be any different?