Without hand-created art to train on, there would be no AI art.
One particular method would not exist. There was AI art before, as AI is a very poor choice of words, as it would include all sorts of machine generated things.
If you would imagine a world where training a model by human made art would be illegal, there are still other ways to make a machine create images. So, there would be "AI art", just not the thing we currently understand by that word.
they are obviously infringing
It is not obvious. If it were, it would be forbidden from day one, everywhere.
When an art student looks at art works and gets inspiration, even imitates techniques and then creates a new work, how can that artist claim copyright. The basis of that work was obviously infringed from other works, just remixed and with some added random bits to create something new.
The only difference is, that the remixing was done inside a human mind.
Maybe read this one here https://en.wikipedia.org/wiki/Derivative_work to understand that it is anything but obvious. Also, copyright ist the right to copy. An AI work that used training data is not a copy.
You can't copyright the knowledge how things look. If you teach a black box how things look and it has the ability to understand a prompt and use that knowledge to create something, that poses a lot of interesting questions with a lot of non obvious and probably conflicting answers.