Digital scales generally work by warping metal pieces and forcing a flex film resistor glued on the piece to be stretched along, causing its resistance to change, thereby breaking the balance of a Wheatstone bridge, creating force-proportional current to occur, which is finally read out with an ADC.

I guess the overall finickiness of that can't be instantly improved by a better microcontroller alone? They drift and have linearlity issues and show temperature dependancy and all that.

Precise body weight measurement at bathrooms is also probably not that important, 500 grams is one full bottle of soda/water; body weights can easily change that much within a single day.

I would suspect that the "real" reason is combination of both. 100-500g can be a "good" compromise for cheap bathroom scales.