I was under the impression that you create a button component and apply the atomic styles in that component. That way you don’t don’t need a “btn” class, you just use the Button component anywhere you need a button.
BUT, then what if you want to add some kind of special behavior to your button that involves subcomponents, e.g. a loading state that conditionally renders a spinner inside the button? Or you want to provide convenience props like rendering an icon before or after the button text? Then a class is not enough.
If you're building a design system/component library, none of these options are simple and there are always tradeoffs.