Because the legal system treats software as a tool and not a legal entity. AI can't hold copyright in the same way your paint brush or copy of photoshop can't hold copyright.
Sure AI is much more effective, but what would it even mean for AI to hold copyright and why would it even be useful to do this?
The legal system serves to provide useful rules for the benefit of society. Even though logically AI tools are more powerful and more self directed than simple programs, there is no reason we would actually want them to own AI because there is no use for a non legal entity to own something.
Sure AI is much more effective, but what would it even mean for AI to hold copyright and why would it even be useful to do this?