Why have so many of these AI models been trained on GPL licensed code? Almost half the controversy could have avoided by ignoring GPL code. I know that even no

Why have so many of these AI models been trained on GPL licensed code? Almost half the controversy could have avoided by ignoring GPL code. I know that even non-copyleft licenses like the Apache and MIT licenses are copyrighted and require attribution however it would have caused far less controversy than training on GPL licensed code.