Why desktop prototypes can't work responsively?

Problem: So I create a responsive website, use auto-layouts, set every constraint up, so when I resize the artboard, it behaves as if I would resize a browser window. Things that need to be centered remain in the center, things that are full width grow with the artboard, just as a responsive website would without a breaking point obviously.

Question: So then, when I start a prototype, why can’t the width and height just assume the size of the browser window? I have already set up everything so that my desktop design would work on every (desktop) size.

It would make viewing a desktop prototype feel like a real website at any size.

Or am I missing a setting that already does that?

A post was merged into an existing topic: Check Responsiveness in Preview (Presentation) mode