Because the question of what data one has the right to use is a very open legal question right now.

There is absolutely nothing illegal about a person examining publicly accessible artwork or text, learning from it, and attempting to reproduce a similar style. AIs are, in essence, doing basically the same thing. However, the sheer difference in time and scale may warrant a different legal treatment. That has not yet been settled, and it will probably take a fair amount of societal debate and new legislation before we have a definite answer.