Because you can target it with CSS and apply styles specific for when the document is viewed without JavaScript enabled.

Is it too much trouble to just override the base style with the js style? As in, don’t specify the base style with “no-js” at all, just the js overrides.

Complicates the base case if you need to undo in it any unwanted styled applied in the no-js case.

Conceptually it's a lot easier to manage if they're fully distinct, or at least if the common bits are explicitly common.

most websites are going to be JS-first. If it makes sense for you to augment your site with JS, then sure knock yourself out and reverse it.

If javascript isn't running in the browser, the script won't run and the class won't change from no-js to js.