Because copyright is a tradeoff. Society wants information to be free. But authors won’t publish if anyone can republish their work for free. Hence the distinction: The ideas you write are not protected - only your expression is.

In a sense, AI changes nothing. Society profits from having AI just as it profits from having people learning from others. In both cases, those who stand on the shoulders of others still make money for themselves. But the economy as a file is richer, too, because people can choose to buy something better now that wasn’t available before.