That would be a good point, as the C in CSS should stand for complicated, but this recommended setting is "auto" (changed from the default of "visible"). Setting it to auto turns on layout containment, style containment, and paint containment for the element. It allows the browser to skip rendering but still keep the elements available to, e.g. tab order. It's like a hybrid of visible and hidden.

https://www.w3.org/TR/css-contain-2/#content-visibility

I think the answer to the GP's question is that the browser does know how to compute this before starting the render pass, but the additions to the spec that allow for placing fences around the layout- and style-related containment was too new for all browsers yet, so the default is to assume all rendering is done, and the HTML/CSS author can opt in to turning this on (which in early days may have had the risk of bugs in implementation or inconsistencies across browsers).