And if they are not used it's more of question of price and other available options and not "the modern architects forgot".

Making what's essentially "an insulated box" is far more universal climate-wise than most of the old methods, because what's good in summer (north-facing windows, good airflow, getting some cold from the ground) is terrible for winter and vice versa. And where it is useful, it IS used, just instead of fireplace having big thermal mass we have floor heating where the concrete floor is the heat storage (and sometimes extra tank of water)

And every method to make it "better" directly competes with "just buy more solar/battery to run heat pump cheaper.