The work is not reproduced in its entirety. Simply using the work in its entirety is not a violation of copyright law, just as reading a book or watching a movie (even if pirated) is not a violation. The reproduction of that work is the violation, and LLMs simply do not store the works in their entirety nor are they capable of reproducing them.
It doesn’t have to be reproduced to be a copyright violation, only used. For example, publishing your Harry Potter fanfic would be infringement. You’re not reproducing the original material in any way, but you’re still heavily depending on it.