ChatGPT probably advises you that because it doesn’t havethe full context. If you told him you are seasoned in CSS and provided better context, I don’t expect it to advise you that.
Meanwhile, ACSS or the Core Framework are fantastic products and are good starting points for someone who is new to this or someone who does not want to pay attention to details.
Also, some Template libraries rely on these frameworks (Brixies, Bricksmaven etc.), and there are a lot of people who prefer doing their work that way.
Overall, ACSS and Core Framework are nice. But if you are serious about this business and you pay attention to details (you should), then you don’t need any framework plugins. It is exactly as @sinanisler pointed out. You do this on your own, and you end up with minimal variables and classes that are needed only for a specific product.
On top of that Bricks will be able to manage your own framework more easily (still in beta).