A human brain is just the summation of all the content it’s ever witnessed, though, both paid and unpaid.
But copyright is entirely artificial. The deal is that the law says you have to pay when you copy a bunch of copyrighted text and reprint it into new pages of a newly bound book. The law also says you don’t have to pay when you are giving commentary on a copyrighted work, or parodying a copyrighted work, or drawing inspiration from a copyrighted work to create something new but still influenced by that copyrighted work. The question for these lawsuits is whether using copyrighted works to train these models and generate new text (or art or music) is infringement of those artificial, human-made, legal rights.
As an example, sound recording copyrights only protect the literal copying of a sound recording. Someone who mimics that copyrighted recording, no matter how perfectly, doesn’t actually infringe on the recording copyright (even if they might infringe on the composition copyright, a separate and distinct copyright). But a literal duplication process of some kind would be infringement.
We can have a debate whether the law draws the line in the correct places, or whether the copyright regime could be improved, and other normative discussion what what the rules should be in the modern world, especially about whether the rules in one area (e.g., the human brain) are consistent with the rules in another area (e.g., a generative AI model). But it’s a separate discussion from what the rules currently are. Under current law, the human brain is currently allowed to perform some types of copying and processing and remixing that some computer programs are not.