Because it can be trivially duplicated
While I agree with your sentiment, designing and manufacturing custom molds for each knob and function (including premium versions) instead of just slapping a screen on the dash does have a cost.> designing and manufacturing custom molds for each knob and function ... dash does have a cost.
Manufacturing car components already involves designing and custom molds, does it not? Compared to the final purchase price, the cost of adding knobs to that stack seems inconsequential.
Has this cost risen?
Why is this so expensive it can't even be put into a premium car today when it used to be ubiquitous in even the cheapest hardware a few decades ago?