Because the scale is different.
You have a mechanism that can regurgitate (digest, remix, emit) without attribution all of the world's code and all of the world's art.
With these systems, you're giving everyone the ability to plagiarize everything, effortlessly and unknowingly. No skill, no effort, no time required. No awareness of the sources of the derivative work.
My work is now your work. Everyone can "write" my code, without ever knowing I wrote it, without ever knowing I existed. Everyone can use my hard work, regurgitated anonymously, stripped of all credit, stripped of all attribution, stripped of all identity and ancestry and citation.
It's a new kind of use not known (or imagined?) when the copyright laws were written.
Training must be opt in, not opt out.
Every artist, every creative individual, must EXPLICITLY OPT IN to having their hard work regurgitated anonymously by Copilot or Dall-E or whatever.
If you want to donate your code or your painting or your music so it can easily be "written" or "painted", in whole or in part, by everyone else, without attribution, then go ahead and opt in.
But if an author or artist does not EXPLICITLY OPT IN, you can't use their creative work to train these systems.
All these code/art washing systems, that absorb and mix and regurgitate the hard work of creative people must be strictly opt in.
That's how the law needs to be.