Hi,
The problem with measuring many ingredients by volume is that this can be inaccurate due to density variations. For example, sugars of various types will vary widely in density, but all contribute simple sugar based on their weight. Adopting a volume oriented system for measuring sugars would be inaccurate since different ones vary quite a bit depending on how finely they are ground.
The same is true for most other non-liquid ingredients, which is why weight is usually used to specify brewing ingredients.
Cheers!
Brad Smith