It's literally a function of the climate control system, and is almost always part of the same set of buttons as temp/fans etc in a car with hard buttons.

When you push the demist button, air is directed to the windscreen, by the same system that directs air to your face or feet.

The use of climate control to de-mist a windscreen is pure implementation detail, and I don’t think the Model 3 even uses climate control for de-misting, instead relying on resistive heaters embedded in the windscreen.

Personally I associate climate control and the temperature setting with personal comfort. But de-misting is a core driving control, and sits along side wipers, lights, fog lights etc. I use all those to drive safely, and expect them to be within easy reach of the driver and readily available. Climate I’m happy to have in a sub-menu, because I can always wait to adjust the temperature if I’m dealing with tricky road conditions, same doesn’t apply to de-fogging equipment (which may, or may not, involve the climate control).