[lifeisstillgood]

They don't - they measure by ratio. 1 cup originally meant just that - whatever cup you had lying around. in an era in the late 19C when few had access to expensive scales, the idea of one level cup became a simple antidote to "butter the size of an egg" recipies. Fannie Farmer's book in 1896 popularised standard ratio measures - see an episode of QI for this and related jokes :-)

[rmc]

Except the sometimes come with units that aren't relative, such as "one egg" or "one teaspoon of sugar".