Hmm. In my opinion solids can very well be quoted in fluid quantities aka volume-wise (by cubic capasity). I think many of us don't own a kitchen scale to convert e.g. 0,5 oz butter into grams or 50 g of butter into ounces (or to pounds). It's much simpler to measure ingredients (approximately) by volume (aka fluid quantities) than to have to search the density (grams per cubic cm) of each ingredient before converting the amount into a familiar weight unit (e.g. by a
convertion chart like this).
All ingredients are "equal" volume-wise = not dependent of their mass/density/weight.
This chart (approximates) is all that is needed for all ingredients world-wide, if you stick to
fluid quantities:
1000 ml = 1 cubic dm = 1 liter (of anything) = 1000 g of water =
a different weight of any other substance, e.g. only
580 g of plain flour
1 teaspoon (of anything) = 5 ml (of anything) = 0,17 fluid oz (of anything) = 0,021 US cups (of anything) = 5 grams/0,17 oz of water (or 4,8 grams of butter or 2,6 grams of plain flour)
1 tablespoon (of anything) = 15 ml (of anything) = 0,5 fl-oz (of anything) = 0,063 US cups (of anything) = 15 grams/0,5 oz of water (or 14,38 grams of butter or 7,8 grams of plain flour)
1 US cup (of anything) = 240 ml (of anything) = 8 fl-oz (of anything) = 240 grams/8 oz of water
1 US pint (of anything) = 480 ml (of anything) = 16 fl-oz (of anything) = 480 grams/16 oz of water
1 UK cup (of anything) = 280 ml (of anything) = 9,47 fl-oz (of anything) = 280 grams/9,47 oz of water
1 UK pint (of anything) = 570 ml (of anything) = 19,3 fl-oz (of anything) = 570 grams/19,3 oz of water
1 US gallon (of anything) = 3750 ml (of anything) = 127 fl-oz (of anything) = 3750 grams/127 oz of water
(1 lb/pound of water = 473 ml of water = 473 grams/16 oz of water); pound isn't a cubic volume
"APF/plain (wheat) flour weighs 0.593 gram per cubic centimeter or 593 kilogram per cubic meter, i.e. density of flour, wheat is equal to 593 kg/m³. In Imperial or US customary measurement system, the density is equal to 37 pound per cubic foot [lb/ft³], or 0.343 ounce per cubic inch [oz/inch³]"
(Source)
It's an endless task to check the weight of each ingredient per cubic meter. However,
grams, ounces or pounds are often used in recipes. In such cases,
weights have to be converted with oz <-> lb <-> gram -converters -
followed with different
ingredient-specific converters,
if cubic measures (like teaspoons, fluid ounces or milliliters) are wanted. Often the context, pics or cooking instructions give an idea of the proportion/balance of quantities, so I don't think it's that important to think about conversions in "ordinary" recipes (excluding soufflés or other delicate dishes with precise quantities). Just have fun and cook! Experimenting is learning
.