I don't have any inside information so perhaps an ex-fruit can shed some light on it, but speculating it seems that Apple's organizational structure implicitly encodes the waterfall pattern.

Apple has divisions for Design, Engineering, etc instead of divisions for products as is more normal. So sometime 20 years ago someone in the Design department designed a spreadsheet app, and they've been stuck with it ever since because Engineering isn't empowered to say to Design that this UI fucking sucks. Even though the app is otherwise regularly updated.

You see the same with Tahoe, when users report that they can't resize the window because the corner radius is so large that it excludes the hit box, Engineering does their best to move the hitbox, but they are not empowered to make the obvious fix which is to reduce the corner radius because that would be a UI change and those only go through waterfall.