What I don't get about all the "if you plan it out first, it gets better" approach is, how did they work before?!

For anything bigger than small size features, I always think about what I do and why I do things. Sometimes in my head, sometimes on paper, a Confluence page or a white board.

I don't really get it. 80 % of software engineering is to figure out what you need and how to achieve this. You check with the stake holders, write down the idea of what you want to do and WHY you want to do it. You do some research.

Last 20 % of the process is coding.

This was always the process. You don't need AI for proper planning and defining your goals.